Allow allocating DMA capable buffers with grant reference module

configname: CONFIG_XEN_GRANT_DMA_ALLOC

Linux Kernel Configuration
└─>Device Drivers
└─>Xen driver support
└─>Allow allocating DMA capable buffers with grant reference module
In linux kernel since version 4.2 (release Date: 2015-08-30)  
Extends grant table module API to allow allocating DMA capable
buffers and mapping foreign grant references on top of it.
The resulting buffer is similar to one allocated by the balloon
driver in that proper memory reservation is made by
({increase|decrease}_reservation and VA mappings are updated if
needed).
This is useful for sharing foreign buffers with HW drivers which
cannot work with scattered buffers provided by the balloon driver,
but require DMAable memory instead.