btrfs: hold a ref on the root in fixup_tree_root_location

Looking up the inode from an arbitrary tree means we need to hold a ref
on that root.
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/inode.c

@@ -5158,6 +5158,10 @@ static int fixup_tree_root_location(struct btrfs_fs_info *fs_info,
 		err = PTR_ERR(new_root);
 		goto out;
 	}
+	if (!btrfs_grab_fs_root(new_root)) {
+		err = -ENOENT;
+		goto out;
+	}
 
 	*sub_root = new_root;
 	location->objectid = btrfs_root_dirid(&new_root->root_item);
@@ -5400,6 +5404,8 @@ struct inode *btrfs_lookup_dentry(struct inode *dir, struct dentry *dentry)
 	} else {
 		inode = btrfs_iget(dir->i_sb, &location, sub_root);
 	}
+	if (root != sub_root)
+		btrfs_put_fs_root(sub_root);
 	srcu_read_unlock(&fs_info->subvol_srcu, index);
 
 	if (!IS_ERR(inode) && root != sub_root) {