btrfs: handle errors from btrfs_dec_ref() properly

In walk_up_proc() we BUG_ON(ret) from btrfs_dec_ref().  This is
incorrect, we have proper error handling here, return the error.
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/extent-tree.c

@@ -5802,7 +5802,10 @@ static noinline int walk_up_proc(struct btrfs_trans_handle *trans,
 				ret = btrfs_dec_ref(trans, root, eb, 1);
 			else
 				ret = btrfs_dec_ref(trans, root, eb, 0);
-			BUG_ON(ret); /* -ENOMEM */
+			if (ret) {
+				btrfs_abort_transaction(trans, ret);
+				return ret;
+			}
 			if (is_fstree(btrfs_root_id(root))) {
 				ret = btrfs_qgroup_trace_leaf_items(trans, eb);
 				if (ret) {