arm64/sysreg: refactor deprecated strncpy

`strncpy` is deprecated for use on NUL-terminated destination strings
[1]. Which seems to be the case here due to the forceful setting of `buf`'s
tail to 0.

A suitable replacement is `strscpy` [2] due to the fact that it
guarantees NUL-termination on its destination buffer argument which is
_not_ the case for `strncpy`!

In this case, we can simplify the logic and also check for any silent
truncation by using `strscpy`'s return value.

This should have no functional change and yet uses a more robust and
less ambiguous interface whilst reducing code complexity.

Link: 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
Suggested-by: Kees Cook <keescook@chromium.org>
Cc: linux-hardening@vger.kernel.org
Signed-off-by: Justin Stitt <justinstitt@google.com>
Link: https://lore.kernel.org/r/20230811-strncpy-arch-arm64-v2-1-ba84eabffadb@google.com
Signed-off-by: Will Deacon <will@kernel.org>

Author: JustinStitt

arch/arm64/kernel/idreg-override.c

@@ -262,9 +262,9 @@ static __init void __parse_cmdline(const char *cmdline, bool parse_aliases)
 		if (!len)
 			return;
 
-		len = min(len, ARRAY_SIZE(buf) - 1);
-		strncpy(buf, cmdline, len);
-		buf[len] = 0;
+		len = strscpy(buf, cmdline, ARRAY_SIZE(buf));
+		if (len == -E2BIG)
+			len = ARRAY_SIZE(buf) - 1;
 
 		if (strcmp(buf, "--") == 0)
 			return;