aio: fix possible invalid memory access when DEBUG is enabled

commit 91d80a84 upstream.

dprintk() shouldn't access @ring after it's unmapped.
Signed-off-by: Zhao Hongjiang <zhaohongjiang@huawei.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
[bwh: Backported to 3.2: keep the second argument to kunmap_atomic()]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>

fs/aio.c

@@ -1112,9 +1112,9 @@ static int aio_read_evt(struct kioctx *ioctx, struct io_event *ent)
 	spin_unlock(&info->ring_lock);
 
 out:
-	kunmap_atomic(ring, KM_USER0);
 	dprintk("leaving aio_read_evt: %d  h%lu t%lu\n", ret,
 		 (unsigned long)ring->head, (unsigned long)ring->tail);
+	kunmap_atomic(ring, KM_USER0);
 	return ret;
 }