btrfs: reset fs_root to NULL on error in open_ctree

While running my error injection script I hit a panic when we tried to
clean up the fs_root when freeing the fs_root.  This is because
fs_info->fs_root == PTR_ERR(-EIO), which isn't great.  Fix this by
setting fs_info->fs_root = NULL; if we fail to read the root.

CC: [email protected] # 4.4+
Reviewed-by: Nikolay Borisov <[email protected]>
Reviewed-by: Johannes Thumshirn <[email protected]>
Reviewed-by: Qu Wenruo <[email protected]>
Signed-off-by: Josef Bacik <[email protected]>
Reviewed-by: David Sterba <[email protected]>
Signed-off-by: David Sterba <[email protected]>

Author: josefbacik

fs/btrfs/disk-io.c

@@ -3200,6 +3200,7 @@ int __cold open_ctree(struct super_block *sb,
 	if (IS_ERR(fs_info->fs_root)) {
 		err = PTR_ERR(fs_info->fs_root);
 		btrfs_warn(fs_info, "failed to read fs tree: %d", err);
+		fs_info->fs_root = NULL;
 		goto fail_qgroup;
 	}