cpumask: mm_cpumask for accessing the struct mm_struct's cpu_vm_mask.

This allows us to change the representation (to a dangling bitmap or
cpumask_var_t) without breaking all the callers: they can use
mm_cpumask() now and won't see a difference as the changes roll into
linux-next.
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

include/linux/mm_types.h

@@ -276,4 +276,7 @@ struct mm_struct {
 #endif
 };
 
+/* Future-safe accessor for struct mm_struct's cpu_vm_mask. */
+#define mm_cpumask(mm) (&(mm)->cpu_vm_mask)
+
 #endif /* _LINUX_MM_TYPES_H */