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>