selftests/bpf: Refactor xdp_context_functional test and bpf program

The existing XDP metadata test works by creating a veth pair and
attaching XDP & TC programs that drop the packet when the condition of
the test isn't fulfilled. The test then pings through the veth pair and
succeeds when the ping comes through.

While this test works great for a veth pair, it is hard to replicate for
tap devices to test the XDP metadata support of them. A similar test for
the tun driver would either involve logic to reply to the ping request,
or would have to capture the packet to check if it was dropped or not.

To make the testing of other drivers easier while still maximizing code
reuse, this commit refactors the existing xdp_context_functional test to
use a test_result map. Instead of conditionally passing or dropping the
packet, the TC program is changed to copy the received metadata into the
value of that single-entry array map. Tests can then verify that the map
value matches the expectation.

This testing logic is easy to adapt to other network drivers as the only
remaining requirement is that there is some way to send a custom
Ethernet packet through it that triggers the XDP & TC programs.

The Ethernet header of that custom packet is all-zero, because it is not
required to be valid for the test to work. The zero ethertype also helps
to filter out packets that are not related to the test and would
otherwise interfere with it.

The payload of the Ethernet packet is used as the test data that is
expected to be passed as metadata from the XDP to the TC program and
written to the map. It has a fixed size of 32 bytes which is a
reasonable size that should be supported by both drivers. Additional
packet headers are not necessary for the test and were therefore skipped
to keep the testing code short.

This new testing methodology no longer requires the veth interfaces to
have IP addresses assigned, therefore these were removed.

Signed-off-by: Marcus Wichelmann <marcus.wichelmann@hetzner-cloud.de>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250305213438.3863922-5-marcus.wichelmann@hetzner-cloud.de

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