librbd: potential seg fault when blacklisting an image client

Fixes: http://tracker.ceph.com/issues/17251
Signed-off-by: Jason Dillaman <dillaman@redhat.com>
(cherry picked from commit 7025fe8)

src/librbd/image_watcher/RewatchRequest.cc

@@ -56,6 +56,7 @@ void RewatchRequest<I>::handle_unwatch(int r) {
   if (r == -EBLACKLISTED) {
     lderr(cct) << "client blacklisted" << dendl;
     finish(r);
+    return;
   } else if (r < 0) {
     lderr(cct) << "failed to unwatch: " << cpp_strerror(r) << dendl;
   }