mailbox: zynqmp: Annotate struct zynqmp_ipi_pdata 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 zynqmp_ipi_pdata.

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

Cc: Jassi Brar <jassisinghbrar@gmail.com>
Cc: Michal Simek <michal.simek@amd.com>
Cc: linux-arm-kernel@lists.infradead.org
Reviewed-by: Justin Stitt <justinstitt@google.com>
Reviewed-by: "Gustavo A. R. Silva" <gustavoars@kernel.org>
Acked-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/20230922175351.work.018-kees@kernel.orgSigned-off-by: Kees Cook <keescook@chromium.org>

drivers/mailbox/zynqmp-ipi-mailbox.c

@@ -108,7 +108,7 @@ struct zynqmp_ipi_pdata {
 	unsigned int method;
 	u32 local_id;
 	int num_mboxes;
-	struct zynqmp_ipi_mbox ipi_mboxes[];
+	struct zynqmp_ipi_mbox ipi_mboxes[] __counted_by(num_mboxes);
 };
 
 static struct device_driver zynqmp_ipi_mbox_driver = {