no matter whether we are compiled as module or if our default of the
net.inet6.ip6.v6only sysctl already matches what we would set.
This avoids unnecessary complications with modules, VIMAGES, INET6 and
the sysctl value, especially considering that most users will use
linux compat as a module.
Discussed with: kib, rwatson (weeks ago)
Reviewed by: rwatson
MFC after: 6 weeks