bpf: Test BPF_SK_REUSEPORT_SELECT_OR_MIGRATE.

This patch adds a test for BPF_SK_REUSEPORT_SELECT_OR_MIGRATE and
removes 'static' from settimeo() in network_helpers.c.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/bpf/20210612123224.12525-12-kuniyu@amazon.co.jp