f2fs: optimize case-insensitive lookups

This patch ports below casefold enhancement patch from ext4 to f2fs

commit 3ae72562 ("ext4: optimize case-insensitive lookups")

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