xen: mark grant mapped pages as foreign

Use the "foreign" page flag to mark pages that have a grant map.  Use
page->private to store information of the grant (the granting domain
and the grant reference).

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

diff --cc arch/x86/xen/p2m.c
Simple merge

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

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