xen: make sure xen_max_p2m_pfn is up to date

Keep xen_max_p2m_pfn up to date with the end of the extra memory
we're adding.  It is possible that it will be too high since memory
may be truncated by a "mem=" option on the kernel command line, but
that won't matter.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
diff --git a/arch/x86/xen/xen-ops.h b/arch/x86/xen/xen-ops.h
index 7c8ab86..d505e98 100644
--- a/arch/x86/xen/xen-ops.h
+++ b/arch/x86/xen/xen-ops.h
@@ -30,6 +30,7 @@
 pgd_t *xen_setup_kernel_pagetable(pgd_t *pgd, unsigned long max_pfn);
 void xen_ident_map_ISA(void);
 void xen_reserve_top(void);
+extern unsigned long xen_max_p2m_pfn;
 
 char * __init xen_memory_setup(void);
 void __init xen_arch_setup(void);