selftests: forwarding: mirror_gre_lib: Support VLAN

Add full_test_span_gre_dir_vlan_ips() and full_test_span_gre_dir_vlan()
to support mirror-to-gre tests that involve VLAN.

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