net-ipv6: add support for sockopt(SOL_IPV6, IPV6_FREEBIND)
So far we've been relying on sockopt(SOL_IP, IP_FREEBIND) being usable
even on IPv6 sockets.
However, it turns out it is perfectly reasonable to want to set freebind
on an AF_INET6 SOCK_RAW socket - but there is no way to set any SOL_IP
socket option on such a socket (they're all blindly errored out).
One use case for this is to allow spoofing src ip on a raw socket
via sendmsg cmsg.