selftests: forwarding: mirror_gre_vlan_bridge_1q: Fix untagged test

The untagged egress test sets up mirroring to {,ip6}gretap such that the
underlay goes through a bridge. Then VLAN flags are manipulated to test
that the traffic leaves the bridge 802.1q-tagged or not, as appropriate.

However, when a neighbor expires at the time that the bridge VLAN is
configured as PVID and egress untagged, the following discovery process
can't finish, because the IP address on H3 is still at the VLAN-tagged
netdevice. This manifests by occasional failures where only several of
the 10 required packets get through.

Therefore, when reconfiguring the VLAN flags, move the IP address to the
appropriate device in the H3 VRF.

In addition to that, take this opportunity to embed an ASCII art diagram
to make the topology move obvious.

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