ceph: warn on missing snap realm

Well, this Shouldn't Happen, so it would be helpful to know the caller when
it does.

Signed-off-by: Sage Weil <sage@newdream.net>

fs/ceph/caps.c

@@ -586,6 +586,7 @@ retry:
 		} else {
 			pr_err("ceph_add_cap: couldn't find snap realm %llx\n",
 			       realmino);
+			WARN_ON(!realm);
 		}
 	}