kernel: sched: fair: Replace rcu_assign_pointer() with RCU_INIT_POINTER()

The use of "rcu_assign_pointer()" is NULLing out the pointer. According to RCU_INIT_POINTER()'s block comment:
"1. This use of RCU_INIT_POINTER() is NULLing out the pointer"
it is better to use it instead of rcu_assign_pointer() because it has a smaller overhead.

The following Coccinelle semantic patch was used:
@@
@@

- rcu_assign_pointer
+ RCU_INIT_POINTER
(..., NULL)

---
 kernel/sched/fair.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index fea7d33..7d4f743 100644
@@ -1759,7 +1759,7 @@ void task_numa_free(struct task_struct *p)
 		list_del(&p->numa_entry);
 		grp->nr_tasks--;
 		spin_unlock_irqrestore(&grp->lock, flags);
-		rcu_assign_pointer(p->numa_group, NULL);
+		RCU_INIT_POINTER(p->numa_group, NULL);
 		put_numa_group(grp);
 	}
 

1.9.1