bcachefs: Check for casefolded dirents in non casefolded dirs

Check for mismatches between casefold dirents and casefold directories.

A mismatch will cause lookups to fail, as we'll be doing the lookup with
the casefolded name, which won't match the non-casefolded dirent, and
vice versa.

Reported-by: Christopher Snowhill <chris@kode54.net>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>