btrfs: handle invalid root reference found in may_destroy_subvol()

The may_destroy_subvol() looks up a root by a key, allowing to do an
inexact search when key->offset is -1.  It's never expected to find such
item, as it would break the allowed range of a root id.
Signed-off-by: David Sterba <dsterba@suse.com>

fs/btrfs/inode.c

@@ -4412,7 +4412,14 @@ static noinline int may_destroy_subvol(struct btrfs_root *root)
 	ret = btrfs_search_slot(NULL, fs_info->tree_root, &key, path, 0, 0);
 	if (ret < 0)
 		goto out;
-	BUG_ON(ret == 0);
+	if (ret == 0) {
+		/*
+		 * Key with offset -1 found, there would have to exist a root
+		 * with such id, but this is out of valid range.
+		 */
+		ret = -EUCLEAN;
+		goto out;
+	}
 
 	ret = 0;
 	if (path->slots[0] > 0) {