Extended lfs_fs_gc to compact metadata, compact_thresh

This extends lfs_fs_gc to now handle three things:

1. Calls mkconsistent if not already consistent
2. Compacts metadata > compact_thresh
3. Populates the block allocator

Which should be all of the janitorial work that can be done without
additional on-disk data structures.

Normally, metadata compaction occurs when an mdir is full, and results in
mdirs that are at most block_size/2.

Now, if you call lfs_fs_gc, littlefs will eagerly compact any mdirs that
exceed the compact_thresh configuration option. Because the resulting
mdirs are at most block_size/2, it only makes sense for compact_thresh to
be >= block_size/2 and <= block_size.

Additionally, there are some special values:

- compact_thresh=0  => defaults to ~88% block_size, may change
- compact_thresh=-1 => disables metadata compaction during lfs_fs_gc

Note that compact_thresh only affects lfs_fs_gc. Normal compactions
still only occur when full.

runners/bench_runner.c

@@ -1321,6 +1321,7 @@ void perm_run(
         .block_cycles       = BLOCK_CYCLES,
         .cache_size         = CACHE_SIZE,
         .lookahead_size     = LOOKAHEAD_SIZE,
+        .compact_thresh     = COMPACT_THRESH,
     };
 
     struct lfs_emubd_config bdcfg = {