This patch allows tcpdiag to support ipv6 work as a module when itself
is also a module.

I was planning for a more extensible method but now I think the effort
would be better directed at a new generic socket netlink interface than
the TCP-specific tcpdiag.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>