socket: skip checking sk_err for recvmmsg(MSG_ERRQUEUE)

recvmmsg does not call ___sys_recvmsg when sk_err is set.
That is fine for normal reads but, for MSG_ERRQUEUE, recvmmsg
should always call ___sys_recvmsg regardless of sk->sk_err to
be able to clear error queue. Otherwise, users are not able to
drain the error queue using recvmmsg.
Signed-off-by: Soheil Hassas Yeganeh <soheil@google.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/socket.c

@@ -2289,10 +2289,12 @@ int __sys_recvmmsg(int fd, struct mmsghdr __user *mmsg, unsigned int vlen,
 	if (!sock)
 		return err;
 
-	err = sock_error(sock->sk);
-	if (err) {
-		datagrams = err;
-		goto out_put;
+	if (likely(!(flags & MSG_ERRQUEUE))) {
+		err = sock_error(sock->sk);
+		if (err) {
+			datagrams = err;
+			goto out_put;
+		}
 	}
 
 	entry = mmsg;