selftests/bpf: Use constants for socket states in sock_fields test

Replace magic numbers in BPF code with constants from bpf.h, so that they
don't require an explanation in the comments.

Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20220317113920.1068535-4-jakub@cloudflare.com