tty: constify return type of tty_name

All users of tty_name pass the result directly to a printf-like
function. This means we can actually let tty_name return the literal
"NULL tty" or tty->name directly, avoiding the strcpy and a lot of
medium-sized stack buffers. In preparation for that, make the return
type const char*.

While at it, we can also constify the tty parameter.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

drivers/tty/tty_io.c

@@ -243,7 +243,7 @@ static void tty_del_file(struct file *file)
  *	Locking: none
  */
 
-char *tty_name(struct tty_struct *tty, char *buf)
+const char *tty_name(const struct tty_struct *tty, char *buf)
 {
 	if (!tty) /* Hmm.  NULL pointer.  That's fun. */
 		strcpy(buf, "NULL tty");

include/linux/tty.h

@@ -421,7 +421,7 @@ static inline struct tty_struct *tty_kref_get(struct tty_struct *tty)
 
 extern int tty_paranoia_check(struct tty_struct *tty, struct inode *inode,
 			      const char *routine);
-extern char *tty_name(struct tty_struct *tty, char *buf);
+extern const char *tty_name(const struct tty_struct *tty, char *buf);
 extern void tty_wait_until_sent(struct tty_struct *tty, long timeout);
 extern int tty_check_change(struct tty_struct *tty);
 extern void __stop_tty(struct tty_struct *tty);