selftests/bpf: cover short IPv4/IPv6 inputs with adjust_room

selftests/bpf: cover short IPv4/IPv6 inputs with adjust_room

Add a selftest covering ETH_HLEN-sized IPv4/IPv6 EtherType inputs for
bpf_prog_test_run_skb().

Reuse a single zero-initialized struct ethhdr eth_hlen and set
eth_hlen.h_proto from the per-test h_proto field.

Also add a dedicated tc_adjust_room program and route the short
IPv4/IPv6 cases to it, so the selftest actually exercises the
bpf_skb_adjust_room() path from the report.

Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Link: https://lore.kernel.org/r/20260408034623.180320-3-sun.jian.kdev@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>