cpumask: tsk_cpumask for accessing the struct task_struct's cpus_allowed.

This allows us to change the representation (to a dangling bitmap or
cpumask_var_t) without breaking all the callers: they can use
tsk_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/sched.h

@@ -1419,6 +1419,9 @@ struct task_struct {
 #endif
 };
 
+/* Future-safe accessor for struct task_struct's cpus_allowed. */
+#define tsk_cpumask(tsk) (&(tsk)->cpus_allowed)
+
 /*
  * Priority of a process goes from 0..MAX_PRIO-1, valid RT
  * priority is 0..MAX_RT_PRIO-1, and SCHED_NORMAL/SCHED_BATCH