dm btree: silence lockdep lock inversion in dm_btree_del()

Allocate memory using GFP_NOIO when deleting a btree.  dm_btree_del()
can be called via an ioctl and we don't want to recurse into the FS or
block layer.
Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Cc: stable@vger.kernel.org

drivers/md/persistent-data/dm-btree.c

@@ -255,7 +255,7 @@ int dm_btree_del(struct dm_btree_info *info, dm_block_t root)
 	int r;
 	struct del_stack *s;
 
-	s = kmalloc(sizeof(*s), GFP_KERNEL);
+	s = kmalloc(sizeof(*s), GFP_NOIO);
 	if (!s)
 		return -ENOMEM;
 	s->info = info;