selftests: Add MACsec VLAN propagation traffic test

selftests: Add MACsec VLAN propagation traffic test

Add VLAN filter propagation tests through offloaded MACsec devices via
actual traffic.

The tests create MACsec tunnels with matching SAs on both endpoints,
stack VLANs on top, and verify connectivity with ping. Covered:
- Offloaded MACsec with VLAN (filters propagate to HW)
- Software MACsec with VLAN (no HW filter propagation)
- Offload on/off toggle and verifying traffic still works

On netdevsim this makes use of the VLAN filter debugfs file to actually
validate that filters are applied/removed correctly.
On real hardware the traffic should validate actual VLAN filter
propagation.

Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Reviewed-by: Sabrina Dubroca <sd@queasysnail.net>
Link: https://patch.msgid.link/20260408115240.1636047-4-cratiu@nvidia.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>