s390/ctcm: replace deprecated strncpy with strscpy

strncpy() is deprecated for use on NUL-terminated destination strings
[1] and as such we should prefer more robust and less ambiguous string
interfaces.

We expect chid to be NUL-terminated based on its use with format
strings:

	CTCM_DBF_TEXT_(SETUP, CTC_DBF_INFO, "%s(%s) %s", CTCM_FUNTAIL,
			chid, ok ? "OK" : "failed");

Moreover, NUL-padding is not required as it is _only_ used in this one
instance with a format string.

Considering the above, a suitable replacement is `strscpy` [2] due to
the fact that it guarantees NUL-termination on the destination buffer
without unnecessarily NUL-padding.

We can also drop the +1 from chid's declaration as we no longer need to
be cautious about leaving a spot for a NUL-byte. Let's use the more
idiomatic strscpy usage of (dest, src, sizeof(dest)) as this more
closely ties the destination buffer to the length.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://manpages.debian.org/testing/linux-manual-4.8/strscpy.9.en.html [2]
Link: KSPP#90
Signed-off-by: Justin Stitt <[email protected]>
Reviewed-by: Thorsten Winkler <[email protected]>
Tested-by: Thorsten Winkler <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Link: https://lore.kernel.org/r/20231023-strncpy-drivers-s390-net-ctcm_main-c-v1-1-265db6e78165@google.com
Signed-off-by: Jakub Kicinski <[email protected]>

drivers/s390/net/ctcm_main.c

@@ -200,13 +200,13 @@ static void channel_free(struct channel *ch)
 static void channel_remove(struct channel *ch)
 {
 	struct channel **c = &channels;
-	char chid[CTCM_ID_SIZE+1];
+	char chid[CTCM_ID_SIZE];
 	int ok = 0;
 
 	if (ch == NULL)
 		return;
 	else
-		strncpy(chid, ch->id, CTCM_ID_SIZE);
+		strscpy(chid, ch->id, sizeof(chid));
 
 	channel_free(ch);
 	while (*c) {