Input: leds - annotate struct input_leds with __counted_by

Prepare for the coming implementation by GCC and Clang of the __counted_by
attribute. Flexible array members annotated with __counted_by can have
their accesses bounds-checked at run-time checking via CONFIG_UBSAN_BOUNDS
(for array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

As found with Coccinelle[1], add __counted_by for struct input_leds.

[1] https://github.com/kees/kernel-tools/blob/trunk/coccinelle/examples/counted_by.cocci

Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230922175031.work.467-kees@kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Author: kees

drivers/input/input-leds.c

@@ -44,7 +44,7 @@ struct input_led {
 struct input_leds {
 	struct input_handle handle;
 	unsigned int num_leds;
-	struct input_led leds[];
+	struct input_led leds[] __counted_by(num_leds);
 };
 
 static enum led_brightness input_leds_brightness_get(struct led_classdev *cdev)