libbpf: Set expected_attach_type for BPF_PROG_TYPE_SK_REUSEPORT.

This commit introduces a new section (sk_reuseport/migrate) and sets
expected_attach_type to two each section in BPF_PROG_TYPE_SK_REUSEPORT
program.

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/20210612123224.12525-11-kuniyu@amazon.co.jp