selftests: router_bridge_pvid_vlan_upper: Add a new selftest

This tests whether addition and deletion of a VLAN upper that coincides
with the current PVID setting throws off forwarding.

This selftests is specifically geared towards offloading drivers. In
particular, mlxsw used to fail this selftest, and an earlier patch in this
patchset fixes the issue. However, there's nothing HW-specific in the test
itself (it absolutely is supposed to pass on SW datapath), and therefore it
is put into the generic forwarding directory.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Danielle Ratson <danieller@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>