selftests: drv-net: xdp: Add rss_hash metadata tests

selftests: drv-net: xdp: Add rss_hash metadata tests

This test loads xdp_metadata.bpf which calls bpf_xdp_metadata_rx_hash() on
incoming packets. The metadata from that packet is then sent to a BPF
map for validation. It borrows structure from xdp.py, reusing common
functions.

The test checks the device's xdp-rx-metadata-features via netlink
before running and skips on devices that do not advertise hash support.
This can be run on veth devices as well as real hardware.

The test is fairly simple and just verifies that a TCP or UDP packet can be
identified as an L4 flow. This minimal test also passes if run on a veth
device.

Signed-off-by: Chris J Arges <carges@cloudflare.com>
Link: https://patch.msgid.link/20260325201139.2501937-7-carges@cloudflare.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>