xen/gntdev: Check and release imported dma-bufs on close

Check if there are any imported dma-bufs left not released by
user-space when grant device's release callback is called and
free those if this is the case. This can happen if user-space
leaks the buffers because of a bug or application has been
terminated for any reason.

Signed-off-by: Oleksandr Andrushchenko <oleksandr_andrushchenko@epam.com>
Reviewed-by: Boris <Ostrovsky@oracle.com>
Signed-off-by: Juergen Gross <jgross@suse.com>