t: Reverted reverted most of LFS_T_MKCONSISTENT

After thinking about this for a bit, there are some compelling
motivations for including an incremental LFS_T_MKCONSISTENT:

- Being able to run incremental LFS_T_MKCONSISTENT traversals in
  parallel with read-only operations is actually quite enticing.

  The only complicated part is maintaining the invalidatable traversal
  state, which already exists with lfsr_traversal_t (except the
  annoying LFS_F_MUTATED bit).

- While it's not really effective to combine LFS_T_MKCONSISTENT and
  LFS_T_LOOKAHEAD traversals, it _is_ possible to combine
  LFS_T_MKCONSISTENT with LFS_T_COMPACT, LFS_T_CKMETA,
  LFS_T_REPAIRMETA (future), etc.

  Really, LFS_T_LOOKAHEAD is the odd one out.

- Making LFS_T_MKCONSISTENT incremental means all filesystem-level
  traversals (except lfsr_mount) can be run incrementally. Which is a
  nice feature to have when O(n = entire fs) risks being very long
  running.

The main downside of LFS_T_MKCONSISTENT (and LFS_T_COMPACT, etc) is that
attempting to run it immediately after mount will likely recursively
trigger a lookahead scan to satisfy block allocation requests -- which
will block the current thread for the duration of the lookahead scan.
But this seems to be more a problem of LFS_T_LOOKAHEAD interacting with
other traversals poorly.

Fortunately, long term, the current plan is to replace the lookahead
buffer with an on-disk block map on disks where the lookahead scan is a
bottleneck. If this gets implemented the problem goes away.

So re-reverting this for now. Worst case we can always re-re-revert this
again in the future. There is already a working implementation, so might
as well see where it goes...

Supporting incremental LFS_T_MKCONSISTENT does add a bit of a code
cost, but there is still some room for deduplicating lfsr_mtree_gc +
lfsr_fs_mkconsistent, which may be interesting:

           code          stack
  before: 35232           2680
  after:  35480 (+0.7%)   2680 (+0.0%)

lfs.h

@@ -179,7 +179,8 @@ enum lfs_traversal_flags {
 //    LFS_T_REPAIRDATA    = 0x0c00, // Repair metadata + data blocks
 
     // internally used flags
-    LFS_F_DIRTY         = 0x1000, // Filesystem has been modified
+    LFS_F_DIRTY         = 0x1000, // Filesystem modified during traversal
+    LFS_F_MUTATED       = 0x4000, // Filesystem modified by traversal
 };