selftests: net: l2tp.sh regression starting with 6.1-rc1

[Andrea Righi]

I noticed that l2tp.sh net selftest is failing in recent kernels with
the following error:

 RTNETLINK answers: Protocol not supported

See also: https://bugs.launchpad.net/bugs/2013014

Apprently the module lt2p_ipv6 is not automatically loaded when the test
is trying to create an l2tp ipv6 tunnel.

I did a bisect and found that the offending commit is this one:

 65b32f801bfb ("uapi: move IPPROTO_L2TP to in.h")

I've temporarily reverted this commit for now, any suggestion on how to
fix this properly?

Thanks,
-Andrea