tty: Fix oops when scanning the polling list for kgdb

Costantino Leandro found a bug in tty_find_polling_driver and provided a
patch that fixed the crash but not the underlying bug. This fixes the
underlying bug where the list walk corrupts the values it is using on a
match but then reuses them if the open fails.
Signed-off-by: NAlan Cox <alan@lxorguk.ukuu.org.uk>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

drivers/char/tty_io.c

@@ -295,7 +295,7 @@ struct tty_driver *tty_find_polling_driver(char *name, int *line)
 	struct tty_driver *p, *res = NULL;
 	int tty_line = 0;
 	int len;
-	char *str;
+	char *str, *stp;
 
 	for (str = name; *str; str++)
 		if ((*str >= '0' && *str <= '9') || *str == ',')
@@ -311,13 +311,14 @@ struct tty_driver *tty_find_polling_driver(char *name, int *line)
 	list_for_each_entry(p, &tty_drivers, tty_drivers) {
 		if (strncmp(name, p->name, len) != 0)
 			continue;
-		if (*str == ',')
-			str++;
-		if (*str == '\0')
-			str = NULL;
+		stp = str;
+		if (*stp == ',')
+			stp++;
+		if (*stp == '\0')
+			stp = NULL;
 
 		if (tty_line >= 0 && tty_line <= p->num && p->ops &&
-		    p->ops->poll_init && !p->ops->poll_init(p, tty_line, str)) {
+		    p->ops->poll_init && !p->ops->poll_init(p, tty_line, stp)) {
 			res = tty_driver_kref_get(p);
 			*line = tty_line;
 			break;