librbd: re-read image header in get_snapc, and call it after snap create and remove
librbd: re-read image header in get_snapc, and call it after snap create and remove

This fixes a bug with image context in which the snap info
in the image header showed fewer snapshots than it should have.

ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph,ajnelson/ceph