bpf: Allow bpf_get_netns_cookie in BPF_PROG_TYPE_SOCK_OPS

We'd like to be able to identify netns from sockops hooks to
accelerate local process communication form different netns.

Signed-off-by: Xu Liu <liuxu623@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210818105820.91894-2-liuxu623@gmail.com