cpumask: use mm_cpumask() wrapper: x86

Makes code futureproof against the impending change to mm->cpu_vm_mask (to be a pointer).

It's also a chance to use the new cpumask_ ops which take a pointer
(the older ones are deprecated, but there's no hurry for arch code).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/arch/x86/xen/mmu.c b/arch/x86/xen/mmu.c
index 093dd59..3bf7b1d 100644
--- a/arch/x86/xen/mmu.c
+++ b/arch/x86/xen/mmu.c
@@ -1165,14 +1165,14 @@
 	/* Get the "official" set of cpus referring to our pagetable. */
 	if (!alloc_cpumask_var(&mask, GFP_ATOMIC)) {
 		for_each_online_cpu(cpu) {
-			if (!cpumask_test_cpu(cpu, &mm->cpu_vm_mask)
+			if (!cpumask_test_cpu(cpu, mm_cpumask(mm))
 			    && per_cpu(xen_current_cr3, cpu) != __pa(mm->pgd))
 				continue;
 			smp_call_function_single(cpu, drop_other_mm_ref, mm, 1);
 		}
 		return;
 	}
-	cpumask_copy(mask, &mm->cpu_vm_mask);
+	cpumask_copy(mask, mm_cpumask(mm));
 
 	/* It's possible that a vcpu may have a stale reference to our
 	   cr3, because its in lazy mode, and it hasn't yet flushed