ceph: keep consistent semantic in fscache related option combination

When specifying multiple fscache related options, the result isn't always
the same as option order, this fix will keep strict consistent meaning
by order.

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