n_tty: Queue buffer work on any available cpu

Scheduling buffer work on the same cpu as the read() thread
limits the parallelism now possible between the receive_buf path
and the n_tty_read() path.
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_tty.c

@@ -198,7 +198,7 @@ static void n_tty_set_room(struct tty_struct *tty)
 		 */
 		WARN_RATELIMIT(test_bit(TTY_LDISC_HALTED, &tty->flags),
 			       "scheduling buffer work for halted ldisc\n");
-		schedule_work(&tty->port->buf.work);
+		queue_work(system_unbound_wq, &tty->port->buf.work);
 	}
 }