librbd: fix snapshot handling

librbd: fix snapshot handling

To ensure consistency, always set the snap context when the header is
updated. If snapid is set, we update librados' snapid when refreshing
the header as well. Also use CEPH_NOSNAP instead of 0 as the default
snapid to prevent confusion. These changes fix snapshot creation
and removal, and prevent writing to a snapshot.

Rollback is fixed by using selfmanaged_snapshot_rollback.

Signed-off-by: Josh Durgin <josh.durgin@dreamhost.com>