[S390] sclp_async: Use kstrtoul_from_user

This patch replaces the code for getting an unsigned long from a
userspace buffer by a simple call to kstroul_from_user.
This makes it easier to read and less error prone.
Signed-off-by: NPeter Huewe <peterhuewe@gmx.de>
Signed-off-by: NHeiko Carstens <heiko.carstens@de.ibm.com>

drivers/s390/char/sclp_async.c

@@ -82,12 +82,9 @@ static int proc_handler_callhome(struct ctl_table *ctl, int write,
 			return -EFAULT;
 	} else {
 		len = *count;
-		rc = copy_from_user(buf, buffer, sizeof(buf));
-		if (rc != 0)
-			return -EFAULT;
-		buf[sizeof(buf) - 1] = '\0';
-		if (strict_strtoul(buf, 0, &val) != 0)
-			return -EINVAL;
+		rc = kstrtoul_from_user(buffer, len, 0, &val);
+		if (rc)
+			return rc;
 		if (val != 0 && val != 1)
 			return -EINVAL;
 		callhome_enabled = val;