selftests/bpf: Test new __sk_buff field gso_size

Analogous to the gso_segs selftests introduced in commit d9ff286a0f
("bpf: allow BPF programs access skb_shared_info->gso_segs field").

Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200303200503.226217-4-willemdebruijn.kernel@gmail.com

tools/testing/selftests/bpf/prog_tests/skb_ctx.c

@@ -14,6 +14,7 @@ void test_skb_ctx(void)
 		.wire_len = 100,
 		.gso_segs = 8,
 		.mark = 9,
+		.gso_size = 10,
 	};
 	struct bpf_prog_test_run_attr tattr = {
 		.data_in = &pkt_v4,