In my recent foray into tcp_diag I discovered some ugly looking ifdef's
on CONFIG_IPV6 that'll break when IPv6 is built as a module.  In order
to fix it, we need to allow tcp_diag to be built as a module.

So here is a start.  This patch move tcp_get_info from tcp_diag.c into
the site of the other caller, tcp.c.
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>