cpumask: use mm_cpumask() wrapper: arm

Makes code futureproof against the impending change to mm->cpu_vm_mask.

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/arm/mm/context.c b/arch/arm/mm/context.c
index fc84fcc..6bda76a 100644
--- a/arch/arm/mm/context.c
+++ b/arch/arm/mm/context.c
@@ -59,6 +59,6 @@
 	}
 	spin_unlock(&cpu_asid_lock);
 
-	mm->cpu_vm_mask = cpumask_of_cpu(smp_processor_id());
+	cpumask_copy(mm_cpumask(mm), cpumask_of(smp_processor_id()));
 	mm->context.id = asid;
 }