relay: fix lock imbalance in relay_late_setup_files

One fail path in relay_late_setup_files() omits
mutex_unlock(&relay_channels_mutex);
Add it.
Signed-off-by: NJiri Slaby <jirislaby@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

kernel/relay.c

@@ -663,8 +663,10 @@ int relay_late_setup_files(struct rchan *chan,
 
 	mutex_lock(&relay_channels_mutex);
 	/* Is chan already set up? */
-	if (unlikely(chan->has_base_filename))
+	if (unlikely(chan->has_base_filename)) {
+		mutex_unlock(&relay_channels_mutex);
 		return -EEXIST;
+	}
 	chan->has_base_filename = 1;
 	chan->parent = parent;
 	curr_cpu = get_cpu();