ceph: new cap message flags indicate if there is pending capsnap

These flags tell mds if there is pending capsnap explicitly.
Without this explicit notification, mds can only conclude if
client has pending capsnap. The method mds use is inefficient
and error-prone.

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