net: openvswitch: Annotate struct dp_meter_instance 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 dp_meter_instance.

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

Cc: Pravin B Shelar <pshelar@ovn.org>
Cc: dev@openvswitch.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230922172858.3822653-10-keescook@chromium.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

net/openvswitch/meter.h

@@ -45,7 +45,7 @@ struct dp_meter {
 struct dp_meter_instance {
 	struct rcu_head rcu;
 	u32 n_meters;
-	struct dp_meter __rcu *dp_meters[];
+	struct dp_meter __rcu *dp_meters[] __counted_by(n_meters);
 };
 
 struct dp_meter_table {