Tweaked lfsr_fs_gc to imply LFS_GC_MTREEONLY based on flags

LFS_GC_MTREEONLY is a rather niche/littlefs-specific flag, and we
probably shouldn't expect users to know when to use it. So now we
automatically switch to LFS_GC_MTREEONLY mode in lfsr_fs_gc if it is
sufficient for accomplishing all pending gc work.

Though currently the only traversal that can be LFS_GC_MTREEONLY is
LFS_GC_MKCONSISTENT...

Note that LFS_GC_MTREEONLY can still be explicitly provided, as it does
change the behavior of LFS_GC_COMPACT and LFS_GC_CKMETA (and combining
LFS_GC_MTREEONLY with LFS_GC_LOOKAHEAD/LFS_GC_CKDATA still asserts).

This adds a bit of code:

           code          stack
  before: 35728           2672
  after:  35740 (+0.0%)   2672 (+0.0%)

lfs.c

@@ -12895,13 +12895,21 @@ int lfsr_fs_gc(lfs_t *lfs, uint32_t flags) {
         return 0;
     }
 
+    // do we really need a full traversal?
+    if (!(lfsr_t_islookahead(flags)
+            || lfsr_t_iscompact(flags)
+            || lfsr_t_isckmeta(flags)
+            || lfsr_t_isckdata(flags))) {
+        flags |= LFS_GC_MTREEONLY;
+    }
+
     // existing traversal?
     if (lfsr_omdir_isopen(lfs, &lfs->gc.o.o)) {
-        // note that we mask out flags! if you change flags mid-traversal,
-        // the result is equivalent to the worst-case set of flags
+        // note that we mask flags (except mtreeonly)! if you change flags
+        // mid-traversal, the result is equivalent to the worst-case set
+        // of flags
         lfs->gc.o.o.flags &= (
-                ~LFS_GC_MTREEONLY
-                    & ~LFS_GC_MKCONSISTENT
+                ~LFS_GC_MKCONSISTENT
                     & ~LFS_GC_LOOKAHEAD
                     & ~LFS_GC_COMPACT
                     & ~LFS_GC_CKMETA