watch_queue: Annotate struct watch_filter 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 watch_filter.

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

Cc: David Howells <[e-mail]>
Cc: Randy Dunlap <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Christian Brauner <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: Siddh Raman Pant <[e-mail]>
Cc: Mauro Carvalho Chehab <[e-mail]>
Cc: Qian Cai <[e-mail]>
Signed-off-by: Kees Cook <[e-mail]>
Tested-by: Siddh Raman Pant <[e-mail]>
Reviewed-by: "Gustavo A. R. Silva" <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Christian Brauner <[e-mail]>

include/linux/watch_queue.h

···
 		DECLARE_BITMAP(type_filter, WATCH_TYPE__NR);
 	};
 	u32			nr_filters;	/* Number of filters */
-	struct watch_type_filter filters[];
+	struct watch_type_filter filters[] __counted_by(nr_filters);
 };
 
 struct watch_queue {