x86/tdx: Replace deprecated strncpy() with strtomem_pad()

x86/tdx: Replace deprecated strncpy() with strtomem_pad()

strncpy() works perfectly here in all cases, however, it is deprecated and
as such we should prefer more robust and less ambiguous string APIs:

    https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings

Let's use strtomem_pad() as this matches the functionality of strncpy()
and is _not_ deprecated.

Signed-off-by: Justin Stitt <justinstitt@google.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Kees Cook <keescook@chromium.org>
Acked-by: Dave Hansen <dave.hansen@linux.intel.com>
Link: https://github.com/KSPP/linux/issues/90
Link: https://lore.kernel.org/r/20231003-strncpy-arch-x86-coco-tdx-tdx-c-v2-1-0bd21174a217@google.com