Deduplicated uninling/split routes in lfsr_mdir_commit

This means no special case for uninling-but-not-splitting, but allows
the entire split route to be deduplicated, simplifying things.

The main downside is that for littlefs to go from a single inlined mdir
filesystem to an mtree filesystem it requires a minimum of 2 mdir
allocations (4 blocks) in all cases. This can be avoided, but I think is
worth the tradeoff since it generally occurs once in a filesystem's
lifetime.

This does make 4 block block devices a bit awkward, but those geometries
are always going to be a bit awkward with littlefs's design. At least
this implementation avoids an unecessary B-tree node where possible...

            code          stack
  before:  22586           2320
  after:   22414 (-0.8%)   2120 (-8.6%)

I _think_, but haven't verified, the significant stack saving comes from
the fact that since there's one route through lfsr_mtree_split_,
lfsr_mtree_split_ can be inlined into lfsr_mdir_commit. This avoids the
marshalling of all its arguments for the function call, which I've
noticed can have a surprising cost.

---

Also fixed a bug where dstart was not updated with mid changes after
splits/drops. The mdir commit cleanup code has a lot of duplication now,
makes me wonder if there's a better way to structure this.

tests/t5_dirs.toml

@@ -6086,7 +6086,7 @@ code = '''
 #
 # This is a useful feature, but it's unintuitive if this should have
 # well-defined behavior, so make sure to test for it
-[cases.t5_dirs_rm_recursive]
+[cases.t5_dirs_recursive_rm]
 defines.N = [1, 2, 4, 8, 16, 32, 64, 128, 256, 512]
 defines.PARENT = [false, true]
 # 0 => don't seek
@@ -6208,7 +6208,7 @@ code = '''
 #
 # This is a useful feature, but it's unintuitive if this should have
 # well-defined behavior, so make sure to test for it
-[cases.t5_dirs_mv_recursive]
+[cases.t5_dirs_recursive_mv]
 defines.N = [1, 2, 4, 8, 16, 32, 64, 128, 256, 512]
 defines.BEFORE = [false, true]
 # 0 => no