selftests: mlxsw: vxlan: Test FDB un/marking on VXLAN join/leave

When a VXLAN device is attached to an offloaded bridge, or when a
front-panel port is attached to a bridge that already has a VXLAN
device, mlxsw should offload the existing offloadable FDB entries.
Similarly when VXLAN device is downed, the FDB entries are unoffloaded,
and the marks thus need to be cleared. Similarly when a front-panel port
device is attached to a bridge with a VXLAN device, or when VLAN flags
are tweaked on a VXLAN port attached to a VLAN-aware bridge.

Test that the replaying / clearing logic works by observing transitions
in presence of offload marks under different scenarios.

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