bcachefs: Fix dirent_casefold_mismatch repair

Instead of simply recreating a mis-casefolded dirent, use the str_hash
repair code, which will rename it if necessary - the dirent might have
been created again with the correct casefolding.

Factor out out bch2_str_hash_repair key() from
__bch2_str_hash_check_key() for the new path to use, and export
bch2_dirent_create_key() as well.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>