ipv4: fib: fix a comment.

In fib_frontend.c, there is a confusing comment; NETLINK_CB(skb).portid does not
refer to a pid of sending process, but rather to a netlink portid.

Signed-off-by: Rami Rosen <ramirose@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>