f2fs: introduce f2fs_casefolded_name slab cache

Add a slab cache: "f2fs_casefolded_name" for memory allocation
of casefold name.

Reviewed-by: Eric Biggers <ebiggers@google.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>