vhost_vdpa: Return -EFAULT if copy_from_user() fails

The copy_to/from_user() functions return the number of bytes which we
weren't able to copy but the ioctl should return -EFAULT if they fail.

Fixes: a127c5bb

 ("vhost-vdpa: fix backend feature ioctls")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Link: https://lore.kernel.org/r/20201023120853.GI282278@mwanda
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
Cc: stable@vger.kernel.org
Acked-by: Jason Wang <jasowang@redhat.com>