trace: introduce missing mutex_unlock()

Impact: fix tracing buffer mutex leak in case of allocation failure

This error was spotted by this semantic patch:

http://www.emn.fr/x-info/coccinelle/mut.html

It looks correct as far as I can tell. Please review.

Signed-off-by: Vegard Nossum <[e-mail]>
Signed-off-by: Steven Rostedt <[e-mail]>
Signed-off-by: Ingo Molnar <[e-mail]>

kernel/trace/ring_buffer.c

···
 		list_del_init(&page->list);
 		free_buffer_page(page);
 	}
+	mutex_unlock(&buffer->mutex);
 	return -ENOMEM;
 }