CVE-2024-26752: 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 ip6appenddata

l2tpip6sendmsg 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 skbqueueempty when deciding how much data to append using ip6appenddata.

However, the code which performed the calculation was incorrect:

ulen = len + skbqueueempty(&sk->skwritequeue) ? 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.