hwmon: (acpi_power_meter) replace open-coded kmemdup_nul

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

Let's refactor this kcalloc() + strncpy() into a kmemdup_nul() which has
more obvious behavior and is less error prone.

Link: https://www.kernel.org/doc/html/latest/process/deprecated.html#strncpy-on-nul-terminated-strings [1]
Link: https://github.com/KSPP/linux/issues/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/20230926-strncpy-drivers-hwmon-acpi_power_meter-c-v5-1-3fc31a9daf99@google.comSigned-off-by: Kees Cook <keescook@chromium.org>

drivers/hwmon/acpi_power_meter.c

@@ -796,14 +796,13 @@ static int read_capabilities(struct acpi_power_meter_resource *resource)
 			goto error;
 		}
 
-		*str = kcalloc(element->string.length + 1, sizeof(u8),
-			       GFP_KERNEL);
+		*str = kmemdup_nul(element->string.pointer, element->string.length,
+				   GFP_KERNEL);
 		if (!*str) {
 			res = -ENOMEM;
 			goto error;
 		}
 
-		strncpy(*str, element->string.pointer, element->string.length);
 		str++;
 	}