net: mana: Annotate struct hwc_dma_buf 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 hwc_dma_buf.

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

Cc: Long Li <longli@microsoft.com>
Cc: Ajay Sharma <sharmaajay@microsoft.com>
Cc: "K. Y. Srinivasan" <kys@microsoft.com>
Cc: Haiyang Zhang <haiyangz@microsoft.com>
Cc: Wei Liu <wei.liu@kernel.org>
Cc: Dexuan Cui <decui@microsoft.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20230922172858.3822653-9-keescook@chromium.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

include/net/mana/hw_channel.h

@@ -121,7 +121,7 @@ struct hwc_dma_buf {
 	u32 gpa_mkey;
 
 	u32 num_reqs;
-	struct hwc_work_request reqs[];
+	struct hwc_work_request reqs[] __counted_by(num_reqs);
 };
 
 typedef void hwc_rx_event_handler_t(void *ctx, u32 gdma_rxq_id,