arp: fix arp_filter on l3slave devices

arp_filter performs an ip_route_output search for arp source address and
checks if output device is the same where the arp request was received,
if it is not, the arp request is not answered.

This route lookup is always done on main route table so l3slave devices
never find the proper route and arp is not answered.

Passing l3mdev_master_ifindex_rcu(dev) return value as oif fixes the
lookup for l3slave devices while maintaining same behavior for non
l3slave devices as this function returns 0 in that case.

Fixes: 613d09b3

 ("net: Use VRF device index for lookups on TX")
Signed-off-by: Miguel Fadon Perlines <mfadon@teldat.com>
Acked-by: David Ahern <dsa@cumulusnetworks.com>
Signed-off-by: David S. Miller <davem@davemloft.net>