tty: Abstract tty buffer work

Introduce API functions to restart and cancel tty buffer work, rather
than manipulate buffer work directly.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
diff --git a/drivers/tty/n_tty.c b/drivers/tty/n_tty.c
index fb8ccbf..1384426 100644
--- a/drivers/tty/n_tty.c
+++ b/drivers/tty/n_tty.c
@@ -201,7 +201,7 @@
 		 */
 		WARN_RATELIMIT(test_bit(TTY_LDISC_HALTED, &tty->flags),
 			       "scheduling buffer work for halted ldisc\n");
-		queue_work(system_unbound_wq, &tty->port->buf.work);
+		tty_buffer_restart_work(tty->port);
 	}
 }