cpumask: use mm_cpumask() wrapper: um

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>

arch/um/include/asm/mmu_context.h

@@ -35,8 +35,8 @@ static inline void switch_mm(struct mm_struct *prev, struct mm_struct *next,
 	unsigned cpu = smp_processor_id();
 
 	if(prev != next){
-		cpu_clear(cpu, prev->cpu_vm_mask);
-		cpu_set(cpu, next->cpu_vm_mask);
+		cpumask_clear_cpu(cpu, mm_cpumask(prev));
+		cpumask_set_cpu(cpu, mm_cpumask(next));
 		if(next != &init_mm)
 			__switch_mm(&next->context.id);
 	}