s390/ipl: Use kstrtobool() instead of strtobool()

strtobool() is the same as kstrtobool().
However, the latter is more used within the kernel.

In order to remove strtobool() and slightly simplify kstrtox.h, switch to
the other function name.

While at it, include the corresponding header file (<linux/kstrtox.h>)

Link: https://lore.kernel.org/all/cover.1667336095.git.christophe.jaillet@wanadoo.fr/Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>

arch/s390/kernel/ipl.c

@@ -12,6 +12,7 @@
 #include <linux/init.h>
 #include <linux/device.h>
 #include <linux/delay.h>
+#include <linux/kstrtox.h>
 #include <linux/panic_notifier.h>
 #include <linux/reboot.h>
 #include <linux/ctype.h>
@@ -779,7 +780,7 @@ static ssize_t reipl_fcp_clear_store(struct kobject *kobj,
 				     struct kobj_attribute *attr,
 				     const char *buf, size_t len)
 {
-	if (strtobool(buf, &reipl_fcp_clear) < 0)
+	if (kstrtobool(buf, &reipl_fcp_clear) < 0)
 		return -EINVAL;
 	return len;
 }
@@ -899,7 +900,7 @@ static ssize_t reipl_nvme_clear_store(struct kobject *kobj,
 				      struct kobj_attribute *attr,
 				      const char *buf, size_t len)
 {
-	if (strtobool(buf, &reipl_nvme_clear) < 0)
+	if (kstrtobool(buf, &reipl_nvme_clear) < 0)
 		return -EINVAL;
 	return len;
 }
@@ -952,7 +953,7 @@ static ssize_t reipl_ccw_clear_store(struct kobject *kobj,
 				     struct kobj_attribute *attr,
 				     const char *buf, size_t len)
 {
-	if (strtobool(buf, &reipl_ccw_clear) < 0)
+	if (kstrtobool(buf, &reipl_ccw_clear) < 0)
 		return -EINVAL;
 	return len;
 }