btrfs: hold a ref on the root in create_subvol

We're creating the new root here, but we should hold the ref until after
we've initialized the inode for it.
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/ioctl.c

@@ -672,10 +672,16 @@ static noinline int create_subvol(struct inode *dir,
 		btrfs_abort_transaction(trans, ret);
 		goto fail;
 	}
+	if (!btrfs_grab_fs_root(new_root)) {
+		ret = -ENOENT;
+		btrfs_abort_transaction(trans, ret);
+		goto fail;
+	}
 
 	btrfs_record_root_in_trans(trans, new_root);
 
 	ret = btrfs_create_subvol_root(trans, new_root, root, new_dirid);
+	btrfs_put_fs_root(new_root);
 	if (ret) {
 		/* We potentially lose an unused inode item here */
 		btrfs_abort_transaction(trans, ret);