Input: Annotate struct ff_device 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 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 ff_device.
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20231006201739.work.350-kees@kernel.orgSigned-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

include/linux/input.h

@@ -562,7 +562,7 @@ struct ff_device {
 
 	int max_effects;
 	struct ff_effect *effects;
-	struct file *effect_owners[];
+	struct file *effect_owners[] __counted_by(max_effects);
 };
 
 int input_ff_create(struct input_dev *dev, unsigned int max_effects);