tty: stop using "delayed_work" in the tty layer

Using delayed-work for tty flip buffers ends up causing us to wait for
the next tick to complete some actions.  That's usually not all that
noticeable, but for certain latency-critical workloads it ends up being
totally unacceptable.

As an extreme case of this, passing a token back-and-forth over a pty
will take two ticks per iteration, so even just a thousand iterations
will take 8 seconds assuming a common 250Hz configuration.

Avoiding the whole delayed work issue brings that ping-pong test-case
down to 0.009s on my machine.

In more practical terms, this latency has been a performance problem for
things like dive computer simulators (simulating the serial interface
using the ptys) and for other environments (Alan mentions a CP/M emulator).

Reported-by: Jef Driesen <jefdriesen@telenet.be>
Acked-by: Greg KH <gregkh@suse.de>
Acked-by: Alan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/kbd_kern.h

@@ -159,7 +159,7 @@ static inline void con_schedule_flip(struct tty_struct *t)
 	if (t->buf.tail != NULL)
 		t->buf.tail->commit = t->buf.tail->used;
 	spin_unlock_irqrestore(&t->buf.lock, flags);
-	schedule_delayed_work(&t->buf.work, 0);
+	schedule_work(&t->buf.work);
 }
 
 #endif