Add support for dma-buf grant access device driver extension

configname: CONFIG_XEN_GNTDEV_DMABUF

Linux Kernel Configuration
└─>Device Drivers
└─>Xen driver support
└─>Add support for dma-buf grant access device driver extension
In linux kernel since version 4.2 (release Date: 2015-08-30)  
Allows userspace processes and kernel modules to use Xen backed
dma-buf implementation. With this extension grant references to
the pages of an imported dma-buf can be exported for other domain
use and grant references coming from a foreign domain can be
converted into a local dma-buf for local export.