xen: disable preempt for leave_lazy_mmu

xen_mc_flush() requires preemption to be disabled for its own sanity,
so disable it while we're flushing.

Signed-off-by: Jeremy Fitzhardinge <jeremy.fitzhardinge@citrix.com>
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index db3802f..b6e5659 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1819,7 +1819,6 @@
 	xen_mark_init_mm_pinned();
 }
 
-
 const struct pv_mmu_ops xen_mmu_ops __initdata = {
 	.pagetable_setup_start = xen_pagetable_setup_start,
 	.pagetable_setup_done = xen_pagetable_setup_done,