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%)

runners/bench_runner.h

@@ -121,6 +121,7 @@ void bench_permutation(size_t i, uint32_t *buffer, size_t size);
     BENCH_DEFINE(INLINE_SIZE,        BLOCK_SIZE/4                           ) \
     BENCH_DEFINE(FRAGMENT_SIZE,      LFS3_MIN(BLOCK_SIZE/8, 512)            ) \
     BENCH_DEFINE(CRYSTAL_THRESH,     BLOCK_SIZE/8                           ) \
+    BENCH_DEFINE(BMAP_SCAN_THRESH,   BLOCK_COUNT/4                          ) \
     BENCH_DEFINE(ERASE_VALUE,        0xff                                   ) \
     BENCH_DEFINE(ERASE_CYCLES,       0                                      ) \
     BENCH_DEFINE(BADBLOCK_BEHAVIOR,  LFS3_EMUBD_BADBLOCK_PROGERROR          ) \
@@ -154,7 +155,8 @@ void bench_permutation(size_t i, uint32_t *buffer, size_t size);
 
 #ifdef LFS3_BMAP
 #define BENCH_BMAP_CFG \
-    .treediff_size      = TREEDIFF_SIZE,
+    .treediff_size      = TREEDIFF_SIZE,        \
+    .bmap_scan_thresh   = BMAP_SCAN_THRESH,
 #else
 #define BENCH_BMAP_CFG
 #endif

runners/test_runner.h

@@ -112,6 +112,7 @@ void test_permutation(size_t i, uint32_t *buffer, size_t size);
     TEST_DEFINE(INLINE_SIZE,        BLOCK_SIZE/4                            ) \
     TEST_DEFINE(FRAGMENT_SIZE,      LFS3_MIN(BLOCK_SIZE/8, 512)             ) \
     TEST_DEFINE(CRYSTAL_THRESH,     BLOCK_SIZE/8                            ) \
+    TEST_DEFINE(BMAP_SCAN_THRESH,   BLOCK_COUNT/4                           ) \
     TEST_DEFINE(ERASE_VALUE,        0xff                                    ) \
     TEST_DEFINE(ERASE_CYCLES,       0                                       ) \
     TEST_DEFINE(BADBLOCK_BEHAVIOR,  LFS3_EMUBD_BADBLOCK_PROGERROR           ) \
@@ -145,7 +146,8 @@ void test_permutation(size_t i, uint32_t *buffer, size_t size);
 
 #ifdef LFS3_BMAP
 #define TEST_BMAP_CFG \
-    .treediff_size      = TREEDIFF_SIZE,
+    .treediff_size      = TREEDIFF_SIZE,        \
+    .bmap_scan_thresh   = BMAP_SCAN_THRESH,
 #else
 #define TEST_BMAP_CFG
 #endif