vp_vdpa: correct the return value when fail to map notification

We forget to assign a error value when we fail to map the notification
during prove. This patch fixes it.

Reported-by: kernel test robot <lkp@intel.com>
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Fixes: 11d8ffed

 ("vp_vdpa: switch to use vp_modern_map_vq_notify()")
Signed-off-by: Jason Wang <jasowang@redhat.com>
Link: https://lore.kernel.org/r/20210624035939.26618-1-jasowang@redhat.com
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>