Btrfs: fix missing check in the btrfs_qgroup_inherit()

The original code forgot to check 'inherit', we should
gurantee that all the qgroups in the struct 'inherit' exist.
Signed-off-by: Wang Shilong <wangsl-fnst@cn.fujitsu.com>
Reviewed-by: Miao Xie <miaox@cn.fujitsu.com>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>

fs/btrfs/qgroup.c

@@ -1412,6 +1412,7 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans,
 	struct btrfs_qgroup *srcgroup;
 	struct btrfs_qgroup *dstgroup;
 	u32 level_size = 0;
+	u64 nums;
 
 	mutex_lock(&fs_info->qgroup_ioctl_lock);
 	if (!fs_info->quota_enabled)
@@ -1422,6 +1423,20 @@ int btrfs_qgroup_inherit(struct btrfs_trans_handle *trans,
 		goto out;
 	}
 
+	if (inherit) {
+		i_qgroups = (u64 *)(inherit + 1);
+		nums = inherit->num_qgroups + 2 * inherit->num_ref_copies +
+		       2 * inherit->num_excl_copies;
+		for (i = 0; i < nums; ++i) {
+			srcgroup = find_qgroup_rb(fs_info, *i_qgroups);
+			if (!srcgroup) {
+				ret = -EINVAL;
+				goto out;
+			}
+			++i_qgroups;
+		}
+	}
+
 	/*
 	 * create a tracking group for the subvol itself
 	 */