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

net/core/filter.c

@@ -4969,6 +4969,12 @@ static int _bpf_getsockopt(struct sock *sk, int level, int optname,
 			goto err_clear;
 
 		switch (optname) {
+		case SO_RCVBUF:
+			*((int *)optval) = sk->sk_rcvbuf;
+			break;
+		case SO_SNDBUF:
+			*((int *)optval) = sk->sk_sndbuf;
+			break;
 		case SO_MARK:
 			*((int *)optval) = sk->sk_mark;
 			break;