xen/gntdev: switch from kcalloc() to kvcalloc()

With sufficient many pages to map gntdev can reach order 9 allocation
sizes. As there is no need to have physically contiguous buffers switch
to kvcalloc() in order to avoid failing allocations.

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