gbmap: Renamed/moved lookahead.bmapped -> gbmap.known

And:

- Tweaked the behavior of gbmap.window/known to _not_ match disk.
  gbmap.known matching disk is what required a separate
  lookahead.bmapped in the first place, but we never use both fields.

- _Don't_ revert gbmap on failed mdir commits!

  This was broken! If we reverted we risked inheriting outdated
  in-flight block information.

  This could be fixed by also zeroing lookahead.bmapped, but would force
  a gbmap rebuild. And why? The only interaction between mdir commit and
  the gbmap is block allocation, which is intentionally allowed to go
  out-of-sync to relax issues like this.

  Note we still revert in lfs3_fs_grow, the new gbmap we create there is
  incompatible with the previous disk size.

As a part of these changes, gbmap.window now behaves roughly the same as
gbmap.known and updates eagerly on block allocation.

This makes lookahead.window and gbmap.window somewhat redundant, but
simplifies the relevant logic (especially due to how lookahead.window
lags behind lookahead.off).

---

A bunch of bugs fell out-of-this, the interactions with lfs3_fs_mkgbmap
and lfs3_fs_grow being especially tricky, but fortunately our testing is
doing a good job.

At least the code changes were minimal, saves a bit of RAM:

                       code          stack          ctx
  no-gbmap before:    37168           2352          684
  no-gbmap after:     37168 (+0.0%)   2352 (+0.0%)  684 (+0.0%)

                       code          stack          ctx
  maybe-gbmap before: 39688           2392          852
  maybe-gbmap after:  39720 (+0.1%)   2376 (-0.7%)  848 (-0.5%)

                       code          stack          ctx
  yes-gbmap before:   39156           2392          852
  yes-gbmap after:    39208 (+0.1%)   2376 (-0.7%)  848 (-0.5%)

tests/test_alloc.toml

@@ -9,7 +9,14 @@
 #
 after = ['test_mtree', 'test_gbmap', 'test_dirs', 'test_files']
 
-defines.FORMAT_BLOCK_COUNT = 'LFS3_IFDEF_YES_GBMAP(3, 2)'
+# Test both with and without the gbmap if available
+defines.GBMAP = [false, true]
+if = '''
+    LFS3_IFDEF_YES_GBMAP(
+        GBMAP,
+        LFS3_IFDEF_GBMAP(true, !GBMAP))
+'''
+defines.FORMAT_BLOCK_COUNT = '(GBMAP) ? 3 : 2'
 
 
 # test that we can alloc
@@ -30,7 +37,10 @@ code = '''
     struct lfs3_cfg cfg = *CFG;
     cfg.block_count = COUNT;
     lfs3_t lfs3;
-    lfs3_format(&lfs3, LFS3_F_RDWR, &cfg) => 0;
+    lfs3_format(&lfs3,
+            LFS3_F_RDWR
+                | ((GBMAP) ? LFS3_IFDEF_GBMAP(LFS3_F_GBMAP, -1) : 0),
+            &cfg) => 0;
     lfs3_mount(&lfs3, LFS3_M_RDWR, &cfg) => 0;
 
     // start allocating
@@ -77,7 +87,10 @@ code = '''
     struct lfs3_cfg cfg = *CFG;
     cfg.block_count = COUNT;
     lfs3_t lfs3;
-    lfs3_format(&lfs3, LFS3_F_RDWR, &cfg) => 0;
+    lfs3_format(&lfs3,
+            LFS3_F_RDWR
+                | ((GBMAP) ? LFS3_IFDEF_GBMAP(LFS3_F_GBMAP, -1) : 0),
+            &cfg) => 0;
     lfs3_mount(&lfs3, LFS3_M_RDWR, &cfg) => 0;
 
     // start allocating
@@ -138,7 +151,10 @@ defines.REMOUNT = [false, true]
 in = 'lfs3.c'
 code = '''
     lfs3_t lfs3;
-    lfs3_format(&lfs3, LFS3_F_RDWR, CFG) => 0;
+    lfs3_format(&lfs3,
+            LFS3_F_RDWR
+                | ((GBMAP) ? LFS3_IFDEF_GBMAP(LFS3_F_GBMAP, -1) : 0),
+            CFG) => 0;
     lfs3_mount(&lfs3, LFS3_M_RDWR, CFG) => 0;
 
     // create this many directories
@@ -305,7 +321,10 @@ in = 'lfs3.c'
 if = '(SIZE*N)/BLOCK_SIZE <= 32'
 code = '''
     lfs3_t lfs3;
-    lfs3_format(&lfs3, LFS3_F_RDWR, CFG) => 0;
+    lfs3_format(&lfs3,
+            LFS3_F_RDWR
+                | ((GBMAP) ? LFS3_IFDEF_GBMAP(LFS3_F_GBMAP, -1) : 0),
+            CFG) => 0;
     lfs3_mount(&lfs3, LFS3_M_RDWR, CFG) => 0;
 
     // create this many files
@@ -480,7 +499,10 @@ in = 'lfs3.c'
 if = '(SIZE*N)/BLOCK_SIZE <= 32'
 code = '''
     lfs3_t lfs3;
-    lfs3_format(&lfs3, LFS3_F_RDWR, CFG) => 0;
+    lfs3_format(&lfs3,
+            LFS3_F_RDWR
+                | ((GBMAP) ? LFS3_IFDEF_GBMAP(LFS3_F_GBMAP, -1) : 0),
+            CFG) => 0;
     lfs3_mount(&lfs3, LFS3_M_RDWR, CFG) => 0;
 
     // create this many files
@@ -670,7 +692,10 @@ code = '''
     struct lfs3_cfg cfg = *CFG;
     cfg.block_count = COUNT;
     lfs3_t lfs3;
-    lfs3_format(&lfs3, LFS3_F_RDWR, &cfg) => 0;
+    lfs3_format(&lfs3,
+            LFS3_F_RDWR
+                | ((GBMAP) ? LFS3_IFDEF_GBMAP(LFS3_F_GBMAP, -1) : 0),
+            &cfg) => 0;
     lfs3_mount(&lfs3, LFS3_M_RDWR, &cfg) => 0;
 
     // create n files repeatedly until we're sure we've wrapped around
@@ -754,7 +779,10 @@ code = '''
     struct lfs3_cfg cfg = *CFG;
     cfg.block_count = COUNT;
     lfs3_t lfs3;
-    lfs3_format(&lfs3, LFS3_F_RDWR, &cfg) => 0;
+    lfs3_format(&lfs3,
+            LFS3_F_RDWR
+                | ((GBMAP) ? LFS3_IFDEF_GBMAP(LFS3_F_GBMAP, -1) : 0),
+            &cfg) => 0;
     lfs3_mount(&lfs3, LFS3_M_RDWR, &cfg) => 0;
 
     // create directories until we run out of space
@@ -837,7 +865,10 @@ code = '''
     struct lfs3_cfg cfg = *CFG;
     cfg.block_count = COUNT;
     lfs3_t lfs3;
-    lfs3_format(&lfs3, LFS3_F_RDWR, &cfg) => 0;
+    lfs3_format(&lfs3,
+            LFS3_F_RDWR
+                | ((GBMAP) ? LFS3_IFDEF_GBMAP(LFS3_F_GBMAP, -1) : 0),
+            &cfg) => 0;
     lfs3_mount(&lfs3, LFS3_M_RDWR, &cfg) => 0;
 
     // create files until we run out of space