cpumask: use mm_cpumask() wrapper: kernel/fork.c

Impact: futureproof

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.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

kernel/fork.c

@@ -284,7 +284,7 @@ static int dup_mmap(struct mm_struct *mm, struct mm_struct *oldmm)
 	mm->free_area_cache = oldmm->mmap_base;
 	mm->cached_hole_size = ~0UL;
 	mm->map_count = 0;
-	cpus_clear(mm->cpu_vm_mask);
+	cpumask_clear(mm_cpumask(mm));
 	mm->mm_rb = RB_ROOT;
 	rb_link = &mm->mm_rb.rb_node;
 	rb_parent = NULL;