wifi: hostap: Add __counted_by for struct prism2_download_data and use struct_size()

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 via CONFIG_UBSAN_BOUNDS (for
array indexing) and CONFIG_FORTIFY_SOURCE (for strcpy/memcpy-family
functions).

While there, use struct_size() helper, instead of the open-coded
version, to calculate the size for the allocation of the whole
flexible structure, including of course, the flexible-array member.

This code was found with the help of Coccinelle, and audited and
fixed manually.

Signed-off-by: Gustavo A. R. Silva <[e-mail]>
Reviewed-by: Kees Cook <[e-mail]>
Signed-off-by: Kalle Valo <[e-mail]>
Link: https://lore.kernel.org/r/ZSRXXvWMMkm7qqRW@work

drivers/net/wireless/intersil/hostap/hostap_download.c

···
 		goto out;
 	}
 
-	dl = kzalloc(sizeof(*dl) + param->num_areas *
-		     sizeof(struct prism2_download_data_area), GFP_KERNEL);
+	dl = kzalloc(struct_size(dl, data, param->num_areas), GFP_KERNEL);
 	if (dl == NULL) {
 		ret = -ENOMEM;
 		goto out;

drivers/net/wireless/intersil/hostap/hostap_wlan.h

···
 		u32 addr; /* wlan card address */
 		u32 len;
 		u8 *data; /* allocated data */
-	} data[];
+	} data[] __counted_by(num_areas);
 };