ipmr: vrf: Find VIFs using the actual device

The skb->dev that is passed into ip_mr_input is
the loX device for VRFs. When we lookup a vif
for this dev, none is found as we do not create
vifs for loopbacks. Instead lookup a vif for the
actual device that the packet was received on,
eg the vlan.

Signed-off-by: Thomas Winter <Thomas.Winter@alliedtelesis.co.nz>
cc: David Ahern <dsa@cumulusnetworks.com>
cc: Nikolay Aleksandrov <nikolay@cumulusnetworks.com>
cc: roopa <roopa@cumulusnetworks.com>
Acked-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>