ipvs: Remove all remaining references to rt->rt_{src,dst}

Remove all remaining references to rt->rt_{src,dst}
by using dest->dst_saddr to cache saddr (used for TUN mode).
For ICMP in FORWARD hook just restrict the rt_mode for NAT
to disable LOCALNODE. All other modes do not allow
IP_VS_RT_MODE_RDR, so we should be safe with the ICMP
forwarding. Using cp->daddr as replacement for rt_dst
is safe for all modes except BYPASS, even when cp->dest is
NULL because it is cp->daddr that is used to assign cp->dest
for sync-ed connections.

Signed-off-by: Julian Anastasov <ja@ssi.bg>
Signed-off-by: David S. Miller <davem@davemloft.net>