dm: free io_barrier after blk_cleanup_queue call

BugLink: http://bugs.launchpad.net/bugs/1642572

commit d09960b0 upstream.

dm_old_request_fn() has paths that access md->io_barrier.  The party
destroying io_barrier should ensure that no future execution of
dm_old_request_fn() is possible.  Move io_barrier destruction to below
blk_cleanup_queue() to ensure this and avoid a NULL pointer crash during
request-based DM device shutdown.
Signed-off-by: Tahsin Erdogan <tahsin@google.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Luis Henriques <luis.henriques@canonical.com>

drivers/md/dm.c

@@ -2260,8 +2260,6 @@ static void cleanup_mapped_device(struct mapped_device *md)
 	if (md->bs)
 		bioset_free(md->bs);
 
-	cleanup_srcu_struct(&md->io_barrier);
-
 	if (md->disk) {
 		spin_lock(&_minor_lock);
 		md->disk->private_data = NULL;
@@ -2273,6 +2271,8 @@ static void cleanup_mapped_device(struct mapped_device *md)
 	if (md->queue)
 		blk_cleanup_queue(md->queue);
 
+	cleanup_srcu_struct(&md->io_barrier);
+
 	if (md->bdev) {
 		bdput(md->bdev);
 		md->bdev = NULL;