xen/efi: refactor deprecated strncpy

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

`efi_loader_signature` has space for 4 bytes. We are copying "Xen" (3 bytes)
plus a NUL-byte which makes 4 total bytes. With that being said, there is
currently not a bug with the current `strncpy()` implementation in terms of
buffer overreads but we should favor a more robust string interface
either way.

A suitable replacement is `strscpy` [2] due to the fact that it guarantees
NUL-termination on the destination buffer while being functionally the
same in this case.

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: https://github.com/KSPP/linux/issues/90
Cc: linux-hardening@vger.kernel.org
Cc: Kees Cook <keescook@chromium.org>
Signed-off-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20230911-strncpy-arch-x86-xen-efi-c-v1-1-96ab2bba2feb@google.comSigned-off-by: Juergen Gross <jgross@suse.com>

arch/x86/xen/efi.c

@@ -138,7 +138,7 @@ void __init xen_efi_init(struct boot_params *boot_params)
 	if (efi_systab_xen == NULL)
 		return;
 
-	strncpy((char *)&boot_params->efi_info.efi_loader_signature, "Xen",
+	strscpy((char *)&boot_params->efi_info.efi_loader_signature, "Xen",
 			sizeof(boot_params->efi_info.efi_loader_signature));
 	boot_params->efi_info.efi_systab = (__u32)__pa(efi_systab_xen);
 	boot_params->efi_info.efi_systab_hi = (__u32)(__pa(efi_systab_xen) >> 32);