selftests: forwarding: gre_multipath: Fix flower filters

The TC filters used in the test do not work with veth devices because the
outer Ethertype is 802.1Q and not IPv4. The test passes with mlxsw
netdevs since the hardware always looks at "The first Ethertype that
does not point to either: VLAN, CNTAG or configurable Ethertype".

Fix this by matching on the VLAN ID instead, but on the ingress side.
The reason why this is not performed at egress is explained in the
commit cited below.

Fixes: 541ad323 ("selftests: forwarding: gre_multipath: Update next-hop statistics match criteria")
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Reported-by: Stephen Suryaputra <ssuryaextr@gmail.com>
Tested-by: Stephen Suryaputra <ssuryaextr@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>