fs: Reserve flag for casefolding

In preparation for including the casefold feature within f2fs, elevate
the EXT4_CASEFOLD_FL flag to FS_CASEFOLD_FL.

Signed-off-by: Daniel Rosenberg <drosen@google.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>