Input: mt - annotate struct input_mt 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_mt.

[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/20230922175036.work.762-kees@kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Author: kees

include/linux/input/mt.h

@@ -47,7 +47,7 @@ struct input_mt {
 	unsigned int flags;
 	unsigned int frame;
 	int *red;
-	struct input_mt_slot slots[];
+	struct input_mt_slot slots[] __counted_by(num_slots);
 };
 
 static inline void input_mt_set_value(struct input_mt_slot *slot,