btrfs: hold a ref on the root in btrfs_check_uuid_tree_entry

We lookup the uuid of arbitrary subvolumes, hold a ref on the root while
we're doing this.
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/volumes.c

@@ -4431,6 +4431,10 @@ static int btrfs_check_uuid_tree_entry(struct btrfs_fs_info *fs_info,
 			ret = 1;
 		goto out;
 	}
+	if (!btrfs_grab_fs_root(subvol_root)) {
+		ret = 1;
+		goto out;
+	}
 
 	switch (type) {
 	case BTRFS_UUID_KEY_SUBVOL:
@@ -4443,7 +4447,7 @@ static int btrfs_check_uuid_tree_entry(struct btrfs_fs_info *fs_info,
 			ret = 1;
 		break;
 	}
-
+	btrfs_put_fs_root(subvol_root);
 out:
 	return ret;
 }