cpuidle: dt: Replace deprecated strncpy() with strscpy()

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

We should prefer more robust and less ambiguous string interfaces.

A suitable replacement is `strscpy` [2] due to the fact that it guarantees
NUL-termination on the destination buffer. With this, we can also drop
the now unnecessary `CPUIDLE_(NAME|DESC)_LEN - 1` pieces.

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
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/20230913-strncpy-drivers-cpuidle-dt_idle_states-c-v1-1-d16a0dbe5658@google.com
Signed-off-by: Kees Cook <keescook@chromium.org>

Author: JustinStitt

drivers/cpuidle/dt_idle_states.c

@@ -84,8 +84,8 @@ static int init_state_node(struct cpuidle_state *idle_state,
 	 *	replace with kstrdup and pointer assignment when name
 	 *	and desc become string pointers
 	 */
-	strncpy(idle_state->name, state_node->name, CPUIDLE_NAME_LEN - 1);
-	strncpy(idle_state->desc, desc, CPUIDLE_DESC_LEN - 1);
+	strscpy(idle_state->name, state_node->name, CPUIDLE_NAME_LEN);
+	strscpy(idle_state->desc, desc, CPUIDLE_DESC_LEN);
 	return 0;
 }