xen: use generic functions instead of xen_{alloc, free}_vm_area()

Replace calls to the Xen-specific xen_alloc_vm_area() and
xen_free_vm_area() functions with the generic equivalent
(alloc_vm_area() and free_vm_area()).

On x86, these were identical already.

Signed-off-by: David Vrabel <david.vrabel@citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
diff --git a/arch/x86/xen/grant-table.c b/arch/x86/xen/grant-table.c
index 49ba9b5..6bbfd7a 100644
--- a/arch/x86/xen/grant-table.c
+++ b/arch/x86/xen/grant-table.c
@@ -71,7 +71,7 @@
 
 	if (shared == NULL) {
 		struct vm_struct *area =
-			xen_alloc_vm_area(PAGE_SIZE * max_nr_gframes);
+			alloc_vm_area(PAGE_SIZE * max_nr_gframes);
 		BUG_ON(area == NULL);
 		shared = area->addr;
 		*__shared = shared;