sctp: support MSG_ERRQUEUE flag in recvmsg()

For some reason sctp_poll() generates EPOLLERR if sk->sk_error_queue
is not empty but recvmsg() can not drain the error queue yet.

This is needed to better support timestamping.

I had to export inet_recv_error(), since sctp
can be compiled as a module.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Marcelo Ricardo Leitner <marcelo.leitner@gmail.com>
Cc: Willem de Bruijn <willemb@google.com>
Acked-by: Xin Long <lucien.xin@gmail.com>
Link: https://lore.kernel.org/r/20231212145550.3872051-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/ipv4/af_inet.c

@@ -1633,6 +1633,7 @@ int inet_recv_error(struct sock *sk, struct msghdr *msg, int len, int *addr_len)
 #endif
 	return -EINVAL;
 }
+EXPORT_SYMBOL(inet_recv_error);
 
 int inet_gro_complete(struct sk_buff *skb, int nhoff)
 {

net/sctp/socket.c

@@ -2099,6 +2099,9 @@ static int sctp_recvmsg(struct sock *sk, struct msghdr *msg, size_t len,
 	pr_debug("%s: sk:%p, msghdr:%p, len:%zd, flags:0x%x, addr_len:%p)\n",
 		 __func__, sk, msg, len, flags, addr_len);
 
+	if (unlikely(flags & MSG_ERRQUEUE))
+		return inet_recv_error(sk, msg, len, addr_len);
+
 	lock_sock(sk);
 
 	if (sctp_style(sk, TCP) && !sctp_sstate(sk, ESTABLISHED) &&