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: NRoberto De Ioris <roberto@unbit.it>
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/xen/setup.c

@@ -42,7 +42,7 @@ char * __init xen_memory_setup(void)
 
 	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