ipv4: icmp: Eliminate remaining uses of rt->rt_src

On input packets, rt->rt_src always equals ip_hdr(skb)->saddr

Anything that mangles or otherwise changes the IP header must
relookup the route found at skb_rtable().  Therefore this
invariant must always hold true.

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