xen/grant-table: add helpers for allocating pages

Add gnttab_alloc_pages() and gnttab_free_pages() to allocate/free pages
suitable to for granted maps.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>

diff --cc drivers/block/xen-blkback/blkback.c
Simple merge

diff --cc drivers/net/xen-netback/interface.c
Simple merge

diff --cc drivers/xen/gntdev.c
Simple merge

diff --cc drivers/xen/grant-table.c
Simple merge

diff --cc drivers/xen/xen-scsiback.c
Simple merge

diff --cc include/xen/grant_table.h
Simple merge