s390/sclp: replace deprecated strncpy with strtomem

Let's move away from using strncpy() as it is deprecated [1].

Instead use strtomem() as `e.id` is already marked as nonstring:
|       char id[4] __nonstring;

We don't need strtomem_pad() because `e` is already memset to 0 --
rendering any additional NUL-padding useless.

Link: https://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>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20231023-strncpy-drivers-s390-char-sclp-c-v1-1-eaeef80522bb@google.com
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>

Author: JustinStitt

drivers/s390/char/sclp.c

@@ -81,7 +81,7 @@ static inline void sclp_trace(int prio, char *id, u32 a, u64 b, bool err)
 	struct sclp_trace_entry e;
 
 	memset(&e, 0, sizeof(e));
-	strncpy(e.id, id, sizeof(e.id));
+	strtomem(e.id, id);
 	e.a = a;
 	e.b = b;
 	debug_event(&sclp_debug, prio, &e, sizeof(e));