selftests: forwarding: vxlan_bridge_1d: fix test failure with br_netfilter enabled

selftests: forwarding: vxlan_bridge_1d: fix test failure with br_netfilter enabled

The test generates VXLAN traffic using mausezahn, where the encapsulated
inner IPv4 packet contains a zero IP header checksum. After VXLAN
decapsulation, such packets do not pass sanity checks in br_netfilter
and are dropped, which causes the test to fail.

Fix this by calculating and setting a valid IPv4 header checksum for the
encapsulated packet generated by mausezahn, so that the packet is accepted
by br_netfilter. Fixed by using the payload_template_calc_checksum() /
payload_template_expand_checksum() helpers that are only available
in v6.3 and newer kernels.

Fixes: a0b61f3d8ebf ("selftests: forwarding: vxlan_bridge_1d: Add an ECN decap test")
Signed-off-by: Aleksei Oladko <aleksey.oladko@virtuozzo.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260213131907.43351-2-aleksey.oladko@virtuozzo.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>