(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.

lfs.h

@@ -183,6 +183,19 @@ enum lfs_traversal_flags {
     LFS_F_MUTATED       = 0x4000, // Filesystem modified by traversal
 };
 
+// GC flags
+enum lfs_gc_flags {
+    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
+// TODO
+//    LFS_GC_REPAIRMETA   = 0x0400, // Repair metadata blocks
+//    LFS_GC_REPAIRDATA   = 0x0800, // Repair metadata + data blocks
+};
+
 
 // Configuration provided during initialization of the littlefs
 struct lfs_config {
@@ -1114,6 +1127,20 @@ lfs_ssize_t lfsr_fs_size(lfs_t *lfs);
 int lfsr_fs_mkconsistent(lfs_t *lfs);
 #endif
 
+#ifndef LFS_READONLY
+// Attempt any janitorial work that may be pending.
+//
+// The exact janitorial work depends on the provided flags. Note that most
+// of this work can also be accomplished incrementally via
+// lfsr_traversal_read.
+//
+// Calling this function is not required, but may allow the offloading of
+// expensive janitorial work to a less time-critical code path.
+//
+// Returns a negative error code on failure.
+int lfsr_fs_gc(lfs_t *lfs, uint32_t flags);
+#endif
+
 #ifndef LFS_READONLY
 // Change the number of blocks used by the filesystem
 //