net/tcp: Set SO_ERROR for poll error in setup

Normally, `SO_ERROR` for disconn events will be set by `tcp_poll_eventhandler`, but when the socket is closed before poll, we should also set the `SO_ERROR`.

On Linux, `tcp_poll` returns `EPOLLERR` event when `sk->sk_err` has value but doesn't let `poll` fail (doesn't set `errno`). https://github.com/torvalds/linux/blob/v6.5/net/ipv4/tcp.c#L594-L596
Note: `sk->sk_err` can be get by socket option `SO_ERROR` on Linux, so `POLLERR` will always be together with `SO_ERROR`.

Common libs like curl may try to read `SO_ERROR` on `POLLERR`.

Signed-off-by: Zhe Weng <[email protected]>

net/tcp/tcp_netpoll.c

@@ -344,6 +344,7 @@ int tcp_pollsetup(FAR struct socket *psock, FAR struct pollfd *fds)
        * exceptional event.
        */
 
+      _SO_CONN_SETERRNO(conn, ENOTCONN);
       eventset |= POLLERR | POLLHUP;
     }
   else if (_SS_ISCONNECTED(conn->sconn.s_flags) &&