tty: convert THROTTLE constants into enum

And make an explicit constant for zero too. This allows for easier type
checking of the parameter.

Note: tty_struct::flow_change is kept as int because include/tty.h
(tty_struct) doesn't see tty/tty.h (this enum).
Signed-off-by: "Jiri Slaby (SUSE)" <jirislaby@kernel.org>
Link: https://lore.kernel.org/r/20230919085156.1578-13-jirislaby@kernel.orgSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty.h

@@ -41,15 +41,20 @@ enum {
 };
 
 /* Values for tty->flow_change */
-#define TTY_THROTTLE_SAFE	1
-#define TTY_UNTHROTTLE_SAFE	2
+enum tty_flow_change {
+	TTY_FLOW_NO_CHANGE,
+	TTY_THROTTLE_SAFE,
+	TTY_UNTHROTTLE_SAFE,
+};
 
-static inline void __tty_set_flow_change(struct tty_struct *tty, int val)
+static inline void __tty_set_flow_change(struct tty_struct *tty,
+					 enum tty_flow_change val)
 {
 	tty->flow_change = val;
 }
 
-static inline void tty_set_flow_change(struct tty_struct *tty, int val)
+static inline void tty_set_flow_change(struct tty_struct *tty,
+				       enum tty_flow_change val)
 {
 	tty->flow_change = val;
 	smp_mb();

drivers/tty/tty_ioctl.c

@@ -104,7 +104,7 @@ void tty_unthrottle(struct tty_struct *tty)
 	if (test_and_clear_bit(TTY_THROTTLED, &tty->flags) &&
 	    tty->ops->unthrottle)
 		tty->ops->unthrottle(tty);
-	tty->flow_change = 0;
+	tty->flow_change = TTY_FLOW_NO_CHANGE;
 	up_write(&tty->termios_rwsem);
 }
 EXPORT_SYMBOL(tty_unthrottle);