btrfs: hold a ref on the root in open_ctree

We lookup the fs_root and put it in our fs_info directly, we should hold
a ref on this root for the lifetime of the fs_info.
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

@@ -1537,6 +1537,7 @@ void btrfs_free_fs_info(struct btrfs_fs_info *fs_info)
 	kfree(fs_info->free_space_root);
 	kfree(fs_info->super_copy);
 	kfree(fs_info->super_for_commit);
+	btrfs_put_fs_root(fs_info->fs_root);
 	kvfree(fs_info);
 }
 
@@ -3206,6 +3207,13 @@ int __cold open_ctree(struct super_block *sb,
 		goto fail_qgroup;
 	}
 
+	if (!btrfs_grab_fs_root(fs_info->fs_root)) {
+		fs_info->fs_root = NULL;
+		err = -ENOENT;
+		btrfs_warn(fs_info, "failed to grab a ref on the fs tree");
+		goto fail_qgroup;
+	}
+
 	if (sb_rdonly(sb))
 		return 0;