vhost: missing __user tags

sparse warns about converting void * to void __user *. This is not new
but only got noticed now that vhost is built on more systems.
This is just a question of __user tags missing in a couple of places,
so fix it up.

Fixes: f8894913 ("vhost: introduce O(1) vq metadata cache")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>