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

@@ -2289,11 +2289,13 @@ int __sys_recvmmsg(int fd, struct mmsghdr __user *mmsg, unsigned int vlen,
 	if (!sock)
 		return err;
 
+	if (likely(!(flags & MSG_ERRQUEUE))) {
 		err = sock_error(sock->sk);
 		if (err) {
 			datagrams = err;
 			goto out_put;
 		}
+	}
 
 	entry = mmsg;
 	compat_entry = (struct compat_mmsghdr __user *)mmsg;