n_tty: Fix EOF push index when termios changes

Commit 40d5e0905a03601d40cd4e46b8690093c2355d03,
'n_tty: Fix EOF push handling' introduced a subtle state
change error wrt EOF push handling when the termios is
changed from non-canonical to canonical mode.

Reset line_start to the current read_tail index, not 0.

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

drivers/tty/n_tty.c

@@ -1758,8 +1758,7 @@ static void n_tty_set_termios(struct tty_struct *tty, struct ktermios *old)
 		canon_change = (old->c_lflag ^ tty->termios.c_lflag) & ICANON;
 	if (canon_change) {
 		bitmap_zero(ldata->read_flags, N_TTY_BUF_SIZE);
-		ldata->line_start = 0;
-		ldata->canon_head = ldata->read_tail;
+		ldata->line_start = ldata->canon_head = ldata->read_tail;
 		ldata->erasing = 0;
 		ldata->lnext = 0;
 	}