ceph: check snap first in ceph_set_acl()

Do the snap check first in ceph_set_acl(), so we can avoid
unnecessary operations when the inode has snap.

Signed-off-by: Chengguang Xu <cgxu519@gmx.com>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>