selftests/bpf: use __bpf_constant_htons in test_prog.c

For some reason, my older GCC (< 4.8) isn't smart enough to optimize the
!__builtin_constant_p() branch in bpf_htons, I see:
  error: implicit declaration of function '__builtin_bswap16'

Let's use __bpf_constant_htons as suggested by Daniel Borkmann.

I tried to use simple htons, but it produces the following:
  test_progs.c:54:17: error: braced-group within expression allowed only
  inside a function
    .eth.h_proto = htons(ETH_P_IP),
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/test_progs.c

@@ -51,10 +51,10 @@ static struct {
 	struct iphdr iph;
 	struct tcphdr tcp;
 } __packed pkt_v4 = {
-	.eth.h_proto = bpf_htons(ETH_P_IP),
+	.eth.h_proto = __bpf_constant_htons(ETH_P_IP),
 	.iph.ihl = 5,
 	.iph.protocol = 6,
-	.iph.tot_len = bpf_htons(MAGIC_BYTES),
+	.iph.tot_len = __bpf_constant_htons(MAGIC_BYTES),
 	.tcp.urg_ptr = 123,
 };
 
@@ -64,9 +64,9 @@ static struct {
 	struct ipv6hdr iph;
 	struct tcphdr tcp;
 } __packed pkt_v6 = {
-	.eth.h_proto = bpf_htons(ETH_P_IPV6),
+	.eth.h_proto = __bpf_constant_htons(ETH_P_IPV6),
 	.iph.nexthdr = 6,
-	.iph.payload_len = bpf_htons(MAGIC_BYTES),
+	.iph.payload_len = __bpf_constant_htons(MAGIC_BYTES),
 	.tcp.urg_ptr = 123,
 };