[NETLINK]: illegal use of pid in rtnetlink

When a netlink message is not related to a netlink socket,
it is issued by kernel socket with pid 0. Netlink "pid" has nothing
to do with current->pid. I called it incorrectly, if it was named "port",
the confusion would be avoided.

Signed-off-by: Alexey Kuznetsov <kuznet@ms2.inr.ac.ru>
Signed-off-by: David S. Miller <davem@davemloft.net>