selftests/bpf: Cover skb metadata access after bpf_skb_adjust_room

selftests/bpf: Cover skb metadata access after bpf_skb_adjust_room

Add a test to verify that skb metadata remains accessible after calling
bpf_skb_adjust_room(), which modifies the packet headroom and can trigger
head reallocation.

The helper expects an Ethernet frame carrying an IP packet so switch test
packet identification by source MAC address since we can no longer rely on
Ethernet proto being set to zero.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Link: https://patch.msgid.link/20251105-skb-meta-rx-path-v4-14-5ceb08a9b37b@cloudflare.com