cpumask: use mm_cpumask() wrapper: mips

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/mips/mm/c-octeon.c

@@ -79,7 +79,7 @@ static void octeon_flush_icache_all_cores(struct vm_area_struct *vma)
 	 * cores it has been used on
 	 */
 	if (vma)
-		mask = vma->vm_mm->cpu_vm_mask;
+		mask = *mm_cpumask(vma->vm_mm);
 	else
 		mask = cpu_online_map;
 	cpu_clear(cpu, mask);