selftests/bpf: Cover read/write to skb metadata at an offset

Exercise r/w access to skb metadata through an offset-adjusted dynptr,
read/write helper with an offset argument, and a slice starting at an
offset.

Also check for the expected errors when the offset is out of bounds.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Reviewed-by: Jesse Brandeburg <jbrandeburg@cloudflare.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://patch.msgid.link/20250814-skb-metadata-thru-dynptr-v7-8-8a39e636e0fb@cloudflare.com