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>

net/ipv4/fib_frontend.c

@@ -974,7 +974,7 @@ static void nl_fib_input(struct sk_buff *skb)
 
 	nl_fib_lookup(frn, tb);
 
-	portid = NETLINK_CB(skb).portid;      /* pid of sending process */
+	portid = NETLINK_CB(skb).portid;      /* netlink portid */
 	NETLINK_CB(skb).portid = 0;        /* from kernel */
 	NETLINK_CB(skb).dst_group = 0;  /* unicast */
 	netlink_unicast(net->ipv4.fibnl, skb, portid, MSG_DONTWAIT);