Btrfs: abort transaction if we don't find the block group

We shouldn't BUG_ON() if there is corruption.  I hit this while testing my block
group patch and the abort worked properly.  Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: Chris Mason <clm@fb.com>

fs/btrfs/extent-tree.c

@@ -3139,21 +3139,21 @@ static int write_one_cache_group(struct btrfs_trans_handle *trans,
 	struct extent_buffer *leaf;
 
 	ret = btrfs_search_slot(trans, extent_root, &cache->key, path, 0, 1);
-	if (ret < 0)
+	if (ret) {
+		if (ret > 0)
+			ret = -ENOENT;
 		goto fail;
-	BUG_ON(ret); /* Corruption */
+	}
 
 	leaf = path->nodes[0];
 	bi = btrfs_item_ptr_offset(leaf, path->slots[0]);
 	write_extent_buffer(leaf, &cache->item, bi, sizeof(cache->item));
 	btrfs_mark_buffer_dirty(leaf);
 	btrfs_release_path(path);
 fail:
-	if (ret) {
+	if (ret)
 		btrfs_abort_transaction(trans, root, ret);
-		return ret;
-	}
-	return 0;
+	return ret;
 
 }