bmap: The initial bmapcache algorithm seems to be working

At least at a proof-of-concept level, there's still a lot of cleanup
needed.

To make things work, lfs3_alloc_ckpoint now takes an mdir, which
provides the target for gbmap gstate updates.

When the bmap is close to empty (configurable via bmap_scan_thresh), we
opportunistically rebuild it during lfs3_alloc_ckpoints. The nice thing
about lfs3_alloc_ckpoint is we know the state of all in-flight blocks,
so rebuilding the bmap just requires traversing the filesystem + in-RAM
state.

We might still fall back to the lookahead buffer, but in theory a well
tuned bmap_scan_thresh can prevent this from becoming a bottleneck (at
the cost of more frequent bmap rebuilds).

---

This is also probably a good time to resume measuring code/ram costs,
though it's worth repeating the above note about the bmap work still
needing cleanup:

             code          stack          ctx
  before:   36840           2368          684
  after:    36920 (+0.2%)   2368 (+0.0%)  684 (+0.0%)

Haha, no, the bmap isn't basically free, it's just an opt-in features.
With -DLFS3_YES_BMAP=1:

             code          stack          ctx
  no bmap:  36920           2368          684
  yes bmap: 38552 (+4.4%)   2472 (+4.4%)  812 (+18.7%)

tests/test_badblocks.toml

@@ -11,6 +11,9 @@ after = [
     'test_compat',
 ]
 
+# TODO bmap workaround?
+ifndef = 'LFS3_BMAP'
+
 
 ## Single-block badblock tests
 #
@@ -58,7 +61,7 @@ code = '''
         lfs3.lookahead.off = 0;
         lfs3.lookahead.known = lfs3_min(8*CFG->lookahead_size,
                 CFG->block_count-2);
-        lfs3_alloc_ckpoint(&lfs3);
+        lfs3_alloc_ckpoint(&lfs3, NULL);
 
         // create a btree
         lfs3_btree_t btree;
@@ -2013,7 +2016,7 @@ code = '''
     lfs3.lookahead.off = 0;
     lfs3.lookahead.known = lfs3_min(8*CFG->lookahead_size,
             CFG->block_count-2);
-    lfs3_alloc_ckpoint(&lfs3);
+    lfs3_alloc_ckpoint(&lfs3, NULL);
 
     // create a btree
     lfs3_btree_t btree;
@@ -3970,7 +3973,7 @@ code = '''
     lfs3.lookahead.off = 0;
     lfs3.lookahead.known = lfs3_min(8*CFG->lookahead_size,
             CFG->block_count-2);
-    lfs3_alloc_ckpoint(&lfs3);
+    lfs3_alloc_ckpoint(&lfs3, NULL);
 
     // create a btree
     lfs3_btree_t btree;