btrfs: hold a ref on the root on the dead roots list

At the point we add a root to the dead roots list we have no open inodes
for that root, so we need to hold a ref on that root to keep it from
disappearing.
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/disk-io.c

@@ -2035,8 +2035,7 @@ void btrfs_free_fs_roots(struct btrfs_fs_info *fs_info)
 
 		if (test_bit(BTRFS_ROOT_IN_RADIX, &gang[0]->state))
 			btrfs_drop_and_free_fs_root(fs_info, gang[0]);
-		else
-			btrfs_put_root(gang[0]);
+		btrfs_put_root(gang[0]);
 	}
 
 	while (1) {

fs/btrfs/transaction.c

@@ -1264,8 +1264,10 @@ void btrfs_add_dead_root(struct btrfs_root *root)
 	struct btrfs_fs_info *fs_info = root->fs_info;
 
 	spin_lock(&fs_info->trans_lock);
-	if (list_empty(&root->root_list))
+	if (list_empty(&root->root_list)) {
+		btrfs_grab_root(root);
 		list_add_tail(&root->root_list, &fs_info->dead_roots);
+	}
 	spin_unlock(&fs_info->trans_lock);
 }
 
@@ -2435,6 +2437,7 @@ int btrfs_clean_one_deleted_snapshot(struct btrfs_root *root)
 	else
 		ret = btrfs_drop_snapshot(root, 1, 0);
 
+	btrfs_put_root(root);
 	return (ret < 0) ? 0 : 1;
 }