xen/arm: Implement domain_get_maximum_gpfn

The function domain_get_maximum_gpfn is returning the maximum gpfn ever
mapped in the guest. We can use d->arch.p2m.max_mapped_gfn for this purpose.

We use this in xenaccess as to avoid the user attempting to set page
permissions on pages which don't exist for the domain, as a non-arch specific
sanity check.

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>

xen/arch/arm/mm.c

@@ -996,7 +996,7 @@ int page_is_ram_type(unsigned long mfn, unsigned long mem_type)
 
 unsigned long domain_get_maximum_gpfn(struct domain *d)
 {
-    return -ENOSYS;
+    return d->arch.p2m.max_mapped_gfn;
 }
 
 void share_xen_page_with_guest(struct page_info *page,