um: virtio_uml: send SET_MEM_TABLE message with the exact size

The rust based userspace vhost devices are very strict and will not
accept the message if it is longer than required. So, only include the
data for the first memory region.

Signed-off-by: Benjamin Berg <benjamin.berg@intel.com>
Link: https://patch.msgid.link/20241103212854.1436046-2-benjamin@sipsolutions.net
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

Author: benzea

arch/um/drivers/virtio_uml.c

@@ -623,7 +623,7 @@ static int vhost_user_set_mem_table(struct virtio_uml_device *vu_dev)
 {
 	struct vhost_user_msg msg = {
 		.header.request = VHOST_USER_SET_MEM_TABLE,
-		.header.size = sizeof(msg.payload.mem_regions),
+		.header.size = offsetof(typeof(msg.payload.mem_regions), regions[1]),
 		.payload.mem_regions.num = 1,
 	};
 	unsigned long reserved = uml_reserved - uml_physmem;