selftests/bpf: use __bpf_constant_htons in test_prog.c for flow dissector

Older GCC (<4.8) isn't smart enough to optimize !__builtin_constant_p()
branch in bpf_htons.

I recently fixed it for pkt_v4 and pkt_v6 in commit a0517a0f
("selftests/bpf: use __bpf_constant_htons in test_prog.c"), but
later added another bunch of bpf_htons in commit bf0f0fd9
("selftests/bpf: add simple BPF_PROG_TEST_RUN examples for flow
dissector").

Fixes: bf0f0fd9 ("selftests/bpf: add simple BPF_PROG_TEST_RUN examples for flow dissector")
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/testing/selftests/bpf/test_progs.c

@@ -1954,7 +1954,7 @@ static struct bpf_flow_keys pkt_v4_flow_keys = {
 	.thoff = sizeof(struct iphdr),
 	.addr_proto = ETH_P_IP,
 	.ip_proto = IPPROTO_TCP,
-	.n_proto = bpf_htons(ETH_P_IP),
+	.n_proto = __bpf_constant_htons(ETH_P_IP),
 };
 
 static struct bpf_flow_keys pkt_v6_flow_keys = {
@@ -1962,7 +1962,7 @@ static struct bpf_flow_keys pkt_v6_flow_keys = {
 	.thoff = sizeof(struct ipv6hdr),
 	.addr_proto = ETH_P_IPV6,
 	.ip_proto = IPPROTO_TCP,
-	.n_proto = bpf_htons(ETH_P_IPV6),
+	.n_proto = __bpf_constant_htons(ETH_P_IPV6),
 };
 
 static void test_flow_dissector(void)