[TCP]: Fix zero port problem in IPv6

When we link a socket into the hash table, we need to make sure that we
set the num/port fields so that it shows us with a non-zero port value
in proc/netlink and on the wire.  This code and comment is copied over
from the IPv4 stack as is.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>