mtpcp not usable with ipv6 if built as module

["Björn JACKE" <bjacke@samba.org>]

Hi,

I know that this is a known issue and https://www.mptcp.dev/setup.html mentions
this but I would like to encourage the developers to fix the problem that mptcp
for ipv6 isn't available if ipv6 is built as a module.

The problem that mptcp doesn't work with ipv6 if built as a module is also not
very obvious when you are working on such a system. It simply does not work,
but no dmesg message or so is mentioning the lack of mptcp support at runtime.

The system where I hit this was as raspi system, where ipv6 is built as a
module due to size reasons, which is reasonable for small memory systems. See
https://github.com/raspberrypi/linux/issues/6518 and
https://github.com/raspberrypi/linux/pull/5487

Maybe this is a bit of a motivation to fix the module build of mptcp with ipv6,
raspberry users (and some more probably) would very much apreciate it.

Cheers
Björn