n_tty: Simplify throttle threshold calculation

The adjustments performed by receive_room() are to ensure a line
termination can always be written to the read buffer. However,
these adjustments are irrelevant to the throttle threshold (because
the threshold < buffer limit).
Signed-off-by: NPeter Hurley <peter@hurleysoftware.com>
Signed-off-by: NGreg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/n_tty.c

@@ -262,7 +262,7 @@ static void n_tty_check_throttle(struct tty_struct *tty)
 	while (1) {
 		int throttled;
 		tty_set_flow_change(tty, TTY_THROTTLE_SAFE);
-		if (receive_room(tty) >= TTY_THRESHOLD_THROTTLE)
+		if (N_TTY_BUF_SIZE - read_cnt(ldata) >= TTY_THRESHOLD_THROTTLE)
 			break;
 		throttled = tty_throttle_safe(tty);
 		if (!throttled)