(Re)implemented lfsr_fs_gc

This just provides a simple, easy-to-call, wrapper over the new
traversal API:

  int lfsr_fs_gc(lfs_t *lfs, uint32_t flags);

The main difference from its previous incarnation, is that lfsr_fs_gc
now takes a flags argument to indicate exactly what gc operations to
perform. This gives the user more control, and may also make the API
more robust towards adding new features:

  LFS_GC_MTREEONLY    = 0x0010, // Only traverse the mtree
  LFS_GC_MKCONSISTENT = 0x0020, // Make the filesystem consistent
  LFS_GC_LOOKAHEAD    = 0x0040, // Populate lookahead buffer
  LFS_GC_COMPACT      = 0x0080, // Compact metadata logs
  LFS_GC_CKMETA       = 0x0100, // Check metadata checksums
  LFS_GC_CKDATA       = 0x0200, // Check metadata + data checksums
  LFS_GC_REPAIRMETA+  = 0x0400, // Repair metadata blocks
  LFS_GC_REPAIRDATA+  = 0x0800, // Repair metadata + data blocks

  + Planned

Alternatively, gc_flags could have been added as a config option. But
making gc_flags a function argument matches other flag APIs (open
mainly), and is slightly more flexible in that it allows a system to do
different gc operations in different system states (though this could
also be accomplished with the hypothetical lfsr_fs_gccfg, which would
probably be good to add anyways).

Worst case, defining a system-wide define that you always pass to
lfsr_fs_gc accomplishes roughly the same thing.

---

This adds a bit more code, mainly to check if we actually need to
traverse, and to make sure traversals accomplish all of the requested
work.

           code          stack
  before: 35448           2680
  after:  35708 (+0.7%)   2672 (-0.3%)

Curiously it also saved a bit of stack, which is a bit silly given this
commit is purely code addition. Apparently something in lfs_alloc and
lfsr_fs_gc is shared, getting uninlined, and messing with the stack
measurement. lfs_alloc is quite sensitive to stack changes after all.

tests/test_badblocks.toml

@@ -5,6 +5,7 @@ after = [
     'test_fwrite',
     'test_forphans',
     'test_traversal',
+    'test_gc',
 ]
 
 

tests/test_powerloss.toml

@@ -9,6 +9,7 @@ after = [
     'test_files',
     'test_forphans',
     'test_traversal',
+    'test_gc',
 ]