xen: set up mmu_ops before trying to set any ptes

xen_setup_stackprotector() ends up trying to set page protections,
so we need to have vm_mmu_ops set up before trying to do so.
Failing to do so causes an early boot crash.

[ Impact: Fix early crash under Xen. ]
Signed-off-by: NJeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>

arch/x86/xen/enlighten.c

@@ -1075,6 +1075,8 @@ asmlinkage void __init xen_start_kernel(void)
 	 * Set up some pagetable state before starting to set any ptes.
 	 */
 
+	xen_init_mmu_ops();
+
 	/* Prevent unwanted bits from being set in PTEs. */
 	__supported_pte_mask &= ~_PAGE_GLOBAL;
 	if (!xen_initial_domain())
@@ -1099,7 +1101,6 @@ asmlinkage void __init xen_start_kernel(void)
 	 */
 	xen_setup_stackprotector();
 
-	xen_init_mmu_ops();
 	xen_init_irq_ops();
 	xen_init_cpuid_mask();