ceph: handle race between vmtruncate and queuing cap snap

It's possible that we create a cap snap while there is pending
vmtruncate (truncate hasn't been processed by worker thread).
We should truncate dirty pages beyond capsnap->size in that case.

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