net: Don't proxy arp respond if iif == rt->dst.dev if private VLAN is disabled

Commit 653241 (net: RFC3069, private VLAN proxy arp support) changed
the behavior of arp proxy to send arp replies back out on the interface
the request came in even if the private VLAN feature is disabled.

Previously we checked rt->dst.dev != skb->dev for in scenarios, when
proxy arp is enabled on for the netdevice and also when individual proxy
neighbour entries have been added.

This patch adds the check back for the pneigh_lookup() scenario.

Signed-off-by: Thomas Graf <tgraf@suug.ch>
Acked-by: Jesper Dangaard Brouer <hawk@comx.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>