selftests: forwarding: vxlan_bridge_1d_ipv6: fix test failure with br_netfilter enabled

selftests: forwarding: vxlan_bridge_1d_ipv6: fix test failure with br_netfilter enabled

The test generates VXLAN traffic using mausezahn, where the encapsulated
inner IPv6 packet has an incorrect payload length set in the IPv6 header.
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 setting the correct IPv6 payload length for the encapsulated
packet generated by mausezahn, so that the packet is accepted
by br_netfilter.

tools/testing/selftests/net/forwarding/vxlan_bridge_1d_ipv6.sh
lines 698-706

              )"00:03:"$(           : Payload length
              )"3a:"$(              : Next header
              )"04:"$(              : Hop limit
              )"$saddr:"$(          : IP saddr
              )"$daddr:"$(          : IP daddr
              )"80:"$(              : ICMPv6.type
              )"00:"$(              : ICMPv6.code
              )"00:"$(              : ICMPv6.checksum
              )

Data after IPv6 header:
• 80: — 1 byte (ICMPv6 type)
• 00: — 1 byte (ICMPv6 code)
• 00: — 1 byte (ICMPv6 checksum, truncated)

Total: 3 bytes → 00:03 is correct. The old value 00:08 did not match
the actual payload size.

Fixes: b07e9957f220 ("selftests: forwarding: Add VxLAN tests with a VLAN-unaware bridge for IPv6")
Signed-off-by: Aleksei Oladko <aleksey.oladko@virtuozzo.com>
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Link: https://patch.msgid.link/20260213131907.43351-3-aleksey.oladko@virtuozzo.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>