leds: ledtrig-backlight: replace strict_strtoul() with kstrtoul()

The usage of strict_strtoul() is not preferred, because
strict_strtoul() is obsolete. Thus, kstrtoul() should be
used.

Signed-off-by: Jingoo Han <jg1.han@samsung.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>
diff --git a/drivers/leds/ledtrig-backlight.c b/drivers/leds/ledtrig-backlight.c
index b941685..c74aff1 100644
--- a/drivers/leds/ledtrig-backlight.c
+++ b/drivers/leds/ledtrig-backlight.c
@@ -76,7 +76,7 @@
 	unsigned long invert;
 	int ret;
 
-	ret = strict_strtoul(buf, 10, &invert);
+	ret = kstrtoul(buf, 10, &invert);
 	if (ret < 0)
 		return ret;