bmap: Simplified bmap configs, reduced to one LFS3_F_GBMAP flag

TLDR: This drops the idea of different bmap strategies/modes, and sorts
out most of the compile-time/runtime conditional bmap interactions.

---

Motivation: Benchmarking (at least up to the 32-bit word limit) has
shown the bmap will unlikely be a significant bottleneck, even on large
disks. The largest disks tend to be NAND, and NAND's ridiculous block
size limits pressure on block allocation.

There are still concerns for areas I haven't measured yet:

- SD/eMMC/FTL - Small blocks, so more pressure on block allocation. In
  theory the logical block size can be artificially increased, but this
  comes with a granularity tradeoff.

- I've only measured throughput, latency is a whole other story.

  However, users have reported lfs3_fs_gc is useful for mitigating this,
  so maybe latency is less of a concern now?

But while there may still be room for improvement via alternative bmap
strategies, the risk a concerning amount of complexity. Yes,
configuration gets more complicated, but the real issue is any bmap
strategies that try to track _deallocations_ (the original idea being
treediffing) risk falling leaking blocks if all cases aren't covered.

The current "bmap cache" strategy strikes a really nice balance where it
reduces _amortized_ block allocation -> ~O(log n) without RAM, while
retaining the safe, bug-resistant, single-source-of-truth properties
that come with lookahead-based allocation.

---

So, long story short, dropping other strategies, and now the presence of
the bmap is a boolean flag.

This is also the first format-specific flag:

- Define LFS3_BMAP to enable the bmap logic, but note by default the
  bmap will still not be used.

- Define LFS3_YES_BMAP to force the bmap to be used.

- With LFS3_BMAP, passing LFS3_F_GBMAP to lfs3_format will include the
  on-disk block-map.

- No flag is needed during mount, the presence of the bmap is determined
  by the on-disk wcompat flags (LFS3_WCOMPAT_GBMAP). This also prevents
  rw mounting if the bmap is not supported, but rdonly mounting is
  allowed.

- Users can check if the bmap is in use via lfs3_fs_stat, which reports
  LFS3_I_GBMAP in the flags field.

There's still some missing pieces, but these will be a bit more
involved:

- lfs3_fs_grow needs to be made bmap aware!

- We probably want something like lfs3_fs_mkgbmap and lfs3_fs_rmgbmap to
  allow converting between bmap backed/not-backed filesystem images.

Code changes minimal:

                code          stack          ctx
  before:      37172           2352          684
  after:       37172 (+0.0%)   2352 (+0.0%)  684 (+0.0%)

                code          stack          ctx
  bmap before: 38844           2456          800
  bmap after:  38852 (+0.0%)   2456 (+0.0%)  800 (+0.0%)

tests/test_gc.toml

@@ -1459,7 +1459,7 @@ code = '''
                 | LFS3_I_COMPACT
                 | LFS3_I_CKMETA
                 | LFS3_I_CKDATA
-                | LFS3_IFDEF_BMAP(LFS3_I_BMAPCACHE, 0)));
+                | LFS3_IFDEF_YES_BMAP(LFS3_I_GBMAP, 0)));
 
     // run gc
     if (AFTER == 0) {
@@ -1575,7 +1575,7 @@ code = '''
                 // note ckdata implies ckmeta
                 | ((!CKMETA && !CKDATA) ? LFS3_I_CKMETA : 0)
                 | ((!CKDATA) ? LFS3_I_CKDATA : 0)
-                | LFS3_IFDEF_BMAP(LFS3_I_BMAPCACHE, 0)));
+                | LFS3_IFDEF_YES_BMAP(LFS3_I_GBMAP, 0)));
 
     lfs3_unmount(&lfs3) => 0;
 '''
@@ -1650,7 +1650,7 @@ code = '''
                 | LFS3_I_COMPACT
                 | LFS3_I_CKMETA
                 | LFS3_I_CKDATA
-                | LFS3_IFDEF_BMAP(LFS3_I_BMAPCACHE, 0)));
+                | LFS3_IFDEF_YES_BMAP(LFS3_I_GBMAP, 0)));
 
     // run gc
     if (AFTER == 0) {
@@ -1766,7 +1766,7 @@ code = '''
                 // note ckdata implies ckmeta
                 | ((!CKMETA && !CKDATA) ? LFS3_I_CKMETA : 0)
                 | ((!CKDATA) ? LFS3_I_CKDATA : 0)
-                | LFS3_IFDEF_BMAP(LFS3_I_BMAPCACHE, 0)));
+                | LFS3_IFDEF_YES_BMAP(LFS3_I_GBMAP, 0)));
 
     // test that we can reset flags with lfs3_fs_unck
     lfs3_fs_unck(&lfs3, GC_FLAGS) => 0;
@@ -1781,7 +1781,7 @@ code = '''
                 // _not_ imply uncking ckmeta
                 | ((!(CKDATA && !CKMETA)) ? LFS3_I_CKMETA : 0)
                 | LFS3_I_CKDATA
-                | LFS3_IFDEF_BMAP(LFS3_I_BMAPCACHE, 0)));
+                | LFS3_IFDEF_YES_BMAP(LFS3_I_GBMAP, 0)));
 
     // run gc
     if (AFTER == 0) {
@@ -1897,7 +1897,7 @@ code = '''
                 // note ckdata implies ckmeta
                 | ((!CKMETA && !CKDATA) ? LFS3_I_CKMETA : 0)
                 | ((!CKDATA) ? LFS3_I_CKDATA : 0)
-                | LFS3_IFDEF_BMAP(LFS3_I_BMAPCACHE, 0)));
+                | LFS3_IFDEF_YES_BMAP(LFS3_I_GBMAP, 0)));
 
     lfs3_unmount(&lfs3) => 0;
 '''