vhost: Use flex_array_size() helper in copy_from_user()

Make use of the flex_array_size() helper to calculate the size of a
flexible array member within an enclosing structure.

This helper offers defense-in-depth against potential integer
overflows, while at the same time makes it explicitly clear that
we are dealing with a flexible array member.
Signed-off-by: NGustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200731130956.GA30525@embeddedorSigned-off-by: NMichael S. Tsirkin <mst@redhat.com>

drivers/vhost/vhost.c

@@ -1405,7 +1405,7 @@ static long vhost_set_memory(struct vhost_dev *d, struct vhost_memory __user *m)
 
 	memcpy(newmem, &mem, size);
 	if (copy_from_user(newmem->regions, m->regions,
-			   mem.nregions * sizeof *m->regions)) {
+			   flex_array_size(newmem, regions, mem.nregions))) {
 		kvfree(newmem);
 		return -EFAULT;
 	}