ipv4: Kill ip_rt_frag_needed().

There is zero point to this function.

It's only real substance is to perform an extremely outdated BSD4.2
ICMP check, which we can safely remove.  If you really have a MTU
limited link being routed by a BSD4.2 derived system, here's a nickel
go buy yourself a real router.

The other actions of ip_rt_frag_needed(), checking and conditionally
updating the peer, are done by the per-protocol handlers of the ICMP
event.

TCP, UDP, et al. have a handler which will receive this event and
transmit it back into the associated route via dst_ops->update_pmtu().

This simplification is important, because it eliminates the one place
where we do not have a proper route context in which to make an
inetpeer lookup.

Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/icmp.c

@@ -673,9 +673,7 @@ static void icmp_unreach(struct sk_buff *skb)
 				LIMIT_NETDEBUG(KERN_INFO pr_fmt("%pI4: fragmentation needed and DF set\n"),
 					       &iph->daddr);
 			} else {
-				info = ip_rt_frag_needed(net, iph,
-							 ntohs(icmph->un.frag.mtu),
-							 skb->dev);
+				info = ntohs(icmph->un.frag.mtu);
 				if (!info)
 					goto out;
 			}