gc: Reverted to only consider ckmeta/ckdata done if not mutated

Two main reasons:

1. If we mount without ckprogs, we do actually have a pretty decent hole
   here where data can be written with errors and go unchecked during
   lfsr_fs_gc.

2. If we're traversing a btree that gets mutated mid-traversal, we're
   kicked entirely off the btree. This means we could miss large ranges
   of btree nodes/data blocks that may not have themselves been mutated.
   Not great.

Worst case, it doesn't hurt to check things again if the filesystem
changes. If this is too much of a bottleneck, you should probably be
running gc in incremental mode anyways, which always starts a new
traversal on ckmeta/ckdata.

Checking for dirty/mutated doesn't really add that much code:

           code          stack
  before: 36240           2680
  after:  36256 (+0.0%)   2680 (+0.0%)

lfs.c

@@ -13129,10 +13129,12 @@ int lfsr_fs_gc(lfs_t *lfs, lfs_soff_t steps, uint32_t flags) {
                         | ((!lfsr_i_isuncompacted(lfs->flags))
                             ? LFS_GC_COMPACT
                             : 0)
-                        // consider our filesystem checked if we complete at
-                        // least one traversal
-                        | LFS_GC_CKMETA
-                        | LFS_GC_CKDATA);
+                        // only consider our filesystem checked if we
+                        // weren't mutated
+                        | ((!lfsr_f_isdirty(lfs->gc.o.o.flags)
+                                && !lfsr_f_ismutated(lfs->gc.o.o.flags))
+                            ? LFS_GC_CKMETA | LFS_GC_CKDATA
+                            : 0));
         }
 
         // decrement steps