f2fs: introduce f2fs_match_name() for cleanup

This patch introduces f2fs_match_name() for cleanup.

BTW, it avoids to fallback to normal comparison once it doesn't
match casefolded name.

Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>