selftests: forwarding: gre_multipath: Update next-hop statistics match criteria

gre_multipath test was using egress vlan_id matching on flows, for the
purpose of collecting next-hops statistics, later to be compared
against configured weights.
As matching on vlan_id on egress direction is not supported on all HW
devices, change the match criteria to use destination IP.

Signed-off-by: Nir Dotan <nird@mellanox.com>
Acked-by: Petr Machata <petrm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>