s390/ipl: refactor deprecated strncpy

`strncpy` is deprecated for use on NUL-terminated destination strings [1].

Use `strscpy` which has the same behavior as `strncpy` here with the
extra safeguard of guaranteeing NUL-termination of destination
strings.  In it's current form, this may result in silent truncation
if the src string has the same size as the destination string.

[hca@linux.ibm.com: use strscpy() instead of strscpy_pad()]
Link: www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings[1]
Link: KSPP#90
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20230811-arch-s390-kernel-v1-1-7edbeeab3809@google.com
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Author: JustinStitt

arch/s390/kernel/ipl.c

@@ -266,7 +266,7 @@ static ssize_t sys_##_prefix##_##_name##_store(struct kobject *kobj,	\
 		struct kobj_attribute *attr,				\
 		const char *buf, size_t len)				\
 {									\
-	strncpy(_value, buf, sizeof(_value) - 1);			\
+	strscpy(_value, buf, sizeof(_value));				\
 	strim(_value);							\
 	return len;							\
 }									\