btrfs: Add assert in __btrfs_del_delalloc_inode

The invariant is that when nr_delalloc_inodes is 0 then the root
mustn't have any inodes on its delalloc inodes list.
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -1754,6 +1754,7 @@ void __btrfs_del_delalloc_inode(struct btrfs_root *root,
 			  &inode->runtime_flags);
 		root->nr_delalloc_inodes--;
 		if (!root->nr_delalloc_inodes) {
+			ASSERT(list_empty(&root->delalloc_inodes));
 			spin_lock(&fs_info->delalloc_root_lock);
 			BUG_ON(list_empty(&root->delalloc_root));
 			list_del_init(&root->delalloc_root);