Arm: tighten translate_get_page()

Permitting writes when the P2M type says "read-only" can't be correct.

Fixes: 1661158723 ("xen/arm: Extend copy_to_guest to support copying from/to guest physical address")
Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Michal Orzel <michal.orzel@amd.com>

xen/arch/arm/guestcopy.c

@@ -44,7 +44,7 @@ static struct page_info *translate_get_page(copy_info_t info, uint64_t addr,
     if ( !page )
         return NULL;
 
-    if ( !p2m_is_ram(p2mt) )
+    if ( write ? p2mt != p2m_ram_rw : !p2m_is_ram(p2mt) )
     {
         put_page(page);
         return NULL;