Unlock the right lock.

The adist_remote_lock is not held in this place, whereas the
adist_recv_list_lock lock is and is picked up during the next iteration.

I found this by annotating our libpthread with Clang's -Wthread-safety
attributes. I will send out a patch for this in the nearby future,
because it's awesome.

MFC after:	2 weeks

contrib/openbsm/bin/auditdistd/sender.c

@@ -643,7 +643,7 @@ recv_thread(void *arg __unused)
 			 * we can use that.
 			 */
 			if (TAILQ_EMPTY(&adist_recv_list)) {
-				rw_unlock(&adist_remote_lock);
+				mtx_unlock(&adist_recv_list_lock);
 				continue;
 			}
 			mtx_unlock(&adist_recv_list_lock);