cpumask: remove cpumask_t from core

Impact: cleanup

struct cpumask is nicer, and we use it to make where we've made code
safe for CONFIG_CPUMASK_OFFSTACK=y.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 74541ca..912823e 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -44,7 +44,7 @@
 static int refcount;
 static struct workqueue_struct *stop_machine_wq;
 static struct stop_machine_data active, idle;
-static const cpumask_t *active_cpus;
+static const struct cpumask *active_cpus;
 static void *stop_machine_work;
 
 static void set_state(enum stopmachine_state newstate)