net: bpf: Use the protocol's set_rcvlowat behavior if there is one

The commit d1361840 ("tcp: fix SO_RCVLOWAT and RCVBUF autotuning")
add one new (struct proto_ops)->set_rcvlowat method so that a protocol
can override the default setsockopt(SO_RCVLOWAT) behavior.

The prior bpf codes don't check and invoke the protos's set_rcvlowat,
now correct it.
Signed-off-by: Gao Feng <gfree.wind@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/core/filter.c

@@ -5063,7 +5063,10 @@ static int __bpf_setsockopt(struct sock *sk, int level, int optname,
 		case SO_RCVLOWAT:
 			if (val < 0)
 				val = INT_MAX;
-			WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);
+			if (sk->sk_socket && sk->sk_socket->ops->set_rcvlowat)
+				ret = sk->sk_socket->ops->set_rcvlowat(sk, val);
+			else
+				WRITE_ONCE(sk->sk_rcvlowat, val ? : 1);
 			break;
 		case SO_MARK:
 			if (sk->sk_mark != val) {