ceph: more accurate statfs

Improve accuracy of statfs reporting for Ceph filesystems comprising
exactly one data pool. In this case, the Ceph monitor can now report
the space usage for the single data pool instead of the global data
for the entire Ceph cluster. Include support for this message in
mon_client and leverage it in ceph/super.

Signed-off-by: Douglas Fuller <dfuller@redhat.com>
Reviewed-by: Yan, Zheng <zyan@redhat.com>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>