librbd: do not invalidate object map when attempting to delete non-existent snapshot

librbd: do not invalidate object map when attempting to delete non-existent snapshot

If duplicate snapshot remove requests are received by the lock owner from a peer
client, the first request will remove the object map. If the second request
arrives while the first is in-progress, it will again attempt to remove the
object map but fail to load it since it's already been deleted. This incorrectly
results in the next object map being flagged as invalid.

Fixes: http://tracker.ceph.com/issues/24516
Signed-off-by: Jason Dillaman <dillaman@redhat.com>