epoll: cleanup: use RCU_INIT_POINTER when nulling

It is always safe to use RCU_INIT_POINTER to NULL a pointer.  This results
in slightly smaller/faster code.
Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/eventpoll.c

@@ -1213,7 +1213,7 @@ static noinline void ep_destroy_wakeup_source(struct epitem *epi)
 {
 	struct wakeup_source *ws = ep_wakeup_source(epi);
 
-	rcu_assign_pointer(epi->ws, NULL);
+	RCU_INIT_POINTER(epi->ws, NULL);
 
 	/*
 	 * wait for ep_pm_stay_awake_rcu to finish, synchronize_rcu is