bpf: Add SO_RCVBUF/SO_SNDBUF in _bpf_getsockopt().

This patch exposes SO_RCVBUF/SO_SNDBUF through bpf_getsockopt().

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.co.jp>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20220104013153.97906-3-kuniyu@amazon.co.jp