mkfs.f2fs: remove IMMUTABLE bit

The immutable bit disallows selinux permission which gives no way to clear it.

Fixes: 8cc4e257ec20 ("mkfs.f2fs: add device aliasing feature")
Reviewed-by: Daeho Jeong <daehojeong@google.com>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>