Drivers: hv: vmbus: Use struct_size() helper in kmalloc()

Make use of the struct_size() helper instead of an open-coded version,
in order to avoid any potential type mistakes or integer overflows that,
in the worst scenario, could lead to heap overflows.

Also, address the following sparse warnings:
drivers/hv/vmbus_drv.c:1132:31: warning: using sizeof on a flexible structure

Link: KSPP#174
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Reviewed-by: Michael Kelley <mikelley@microsoft.com>
Link: https://lore.kernel.org/r/20220125180131.GA67746@embeddedor
Signed-off-by: Wei Liu <wei.liu@kernel.org>

Author: GustavoARSilva

drivers/hv/vmbus_drv.c

@@ -1129,7 +1129,7 @@ void vmbus_on_msg_dpc(unsigned long data)
 	}
 
 	if (entry->handler_type	== VMHT_BLOCKING) {
-		ctx = kmalloc(sizeof(*ctx) + payload_size, GFP_ATOMIC);
+		ctx = kmalloc(struct_size(ctx, msg.payload, payload_size), GFP_ATOMIC);
 		if (ctx == NULL)
 			return;