wifi: brcmfmac: Annotate struct brcmf_gscan_config 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 brcmf_gscan_config.

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

Cc: Arend van Spriel <aspriel@gmail.com>
Cc: Franky Lin <franky.lin@broadcom.com>
Cc: Hante Meuleman <hante.meuleman@broadcom.com>
Cc: Kalle Valo <kvalo@kernel.org>
Cc: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Cc: Hector Martin <marcan@marcan.st>
Cc: Ryohei Kondo <ryohei.kondo@cypress.com>
Cc: Hans de Goede <hdegoede@redhat.com>
Cc: linux-wireless@vger.kernel.org
Cc: brcm80211-dev-list.pdl@broadcom.com
Cc: SHA-cyfmac-dev-list@infineon.com
Signed-off-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Signed-off-by: Kalle Valo <kvalo@kernel.org>
Link: https://lore.kernel.org/r/20230915200542.never.417-kees@kernel.org

drivers/net/wireless/broadcom/brcm80211/brcmfmac/fwil_types.h

@@ -1209,7 +1209,7 @@ struct brcmf_gscan_config {
 	u8 count_of_channel_buckets;
 	u8 retry_threshold;
 	__le16  lost_ap_window;
-	struct brcmf_gscan_bucket_config bucket[];
+	struct brcmf_gscan_bucket_config bucket[] __counted_by(count_of_channel_buckets);
 };
 
 /**