ceph: return errors from posix_acl_equiv_mode() correctly

In order to return correct error code should replace variable ret
using err in error case.

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