Btrfs: return ENOMEM rather than use BUG_ON when btrfs_alloc_path fails

Return ENOMEM rather trigger BUG_ON, fix it.
Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Reviewed-by: Zach Brown <zab@redhat.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/qgroup.c

@@ -620,7 +620,9 @@ static int update_qgroup_limit_item(struct btrfs_trans_handle *trans,
 	key.offset = qgroupid;
 
 	path = btrfs_alloc_path();
-	BUG_ON(!path);
+	if (!path)
+		return -ENOMEM;
+
 	ret = btrfs_search_slot(trans, root, &key, path, 0, 1);
 	if (ret > 0)
 		ret = -ENOENT;
@@ -661,7 +663,9 @@ static int update_qgroup_info_item(struct btrfs_trans_handle *trans,
 	key.offset = qgroup->qgroupid;
 
 	path = btrfs_alloc_path();
-	BUG_ON(!path);
+	if (!path)
+		return -ENOMEM;
+
 	ret = btrfs_search_slot(trans, root, &key, path, 0, 1);
 	if (ret > 0)
 		ret = -ENOENT;
@@ -702,7 +706,9 @@ static int update_qgroup_status_item(struct btrfs_trans_handle *trans,
 	key.offset = 0;
 
 	path = btrfs_alloc_path();
-	BUG_ON(!path);
+	if (!path)
+		return -ENOMEM;
+
 	ret = btrfs_search_slot(trans, root, &key, path, 0, 1);
 	if (ret > 0)
 		ret = -ENOENT;