n_tty: Reset lnext if canonical mode changes

lnext escapes the next input character as a literal, and must
be reset when canonical mode changes (to avoid misinterpreting
a special character as a literal if canonical mode is changed
back again).

lnext is specifically not reset on a buffer flush so as to avoid
misinterpreting the next input character as a special character.

Signed-off-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_tty.c

@@ -1574,6 +1574,7 @@ static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)
 		bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);
 		ldata->canon_head = ldata->read_tail;
 		ldata->erasing = 0;
+		ldata->lnext = 0;
 	}
 
 	if (canon_change && !L_ICANON(tty) && read_cnt(ldata))