tty: Fix race in the flush for some ldiscs

If you issue an ioctl to flush a tty as the line discipline is changing or
otherwise unplugged you can get a crash. The bug is very old but the rest
of the BKL lock dropping and some very "good" luck on Ingo's part caught
an example.

Use the correct ldisc_ref form so that we wait for the ldisc change to
complete and then flush

Signed-off-by: Alan Cox <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

drivers/char/tty_ioctl.c

···
 	if (retval)
 		return retval;
 
-	ld = tty_ldisc_ref(tty);
+	ld = tty_ldisc_ref_wait(tty);
 	switch (arg) {
 	case TCIFLUSH:
 		if (ld && ld->ops->flush_buffer)