xen: fix for xen guest with mem > 3.7G

PFN_PHYS() can truncate large addresses unless its passed a suitable
large type.  This is fixed more generally in the patch series
introducing phys_addr_t, but we need a short-term fix to solve a
Xen regression reported by Roberto De Ioris.

Reported-by: Roberto De Ioris <roberto@unbit.it>
Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/xen/setup.c b/arch/x86/xen/setup.c
index b6acc3a..d679010 100644
--- a/arch/x86/xen/setup.c
+++ b/arch/x86/xen/setup.c
@@ -42,7 +42,7 @@
 
 	e820.nr_map = 0;
 
-	e820_add_region(0, PFN_PHYS(max_pfn), E820_RAM);
+	e820_add_region(0, PFN_PHYS((u64)max_pfn), E820_RAM);
 
 	/*
 	 * Even though this is normal, usable memory under Xen, reserve