Input: evdev - annotate struct evdev_client 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 evdev_client.

[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/20230922175027.work.563-kees@kernel.org
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>

Author: kees

drivers/input/evdev.c

@@ -50,7 +50,7 @@ struct evdev_client {
 	bool revoked;
 	unsigned long *evmasks[EV_CNT];
 	unsigned int bufsize;
-	struct input_event buffer[];
+	struct input_event buffer[] __counted_by(bufsize);
 };
 
 static size_t evdev_get_mask_cnt(unsigned int type)