l2tp: pass correct message length to ip6_append_data

In the Linux kernel, the following vulnerability has been resolved:

l2tp: pass correct message length to ip6_append_data

l2tp_ip6_sendmsg needs to avoid accounting for the transport header

twice when splicing more data into an already partially-occupied skbuff.

To manage this, we check whether the skbuff contains data using

skb_queue_empty when deciding how much data to append using

ip6_append_data.

However, the code which performed the calculation was incorrect:

ulen = len + skb_queue_empty(&sk->sk_write_queue) ? transhdrlen : 0;

...due to C operator precedence, this ends up setting ulen to

transhdrlen for messages with a non-zero length, which results in

corrupted packets on the wire.

Add parentheses to correct the calculation in line with the original

intent.