annotate poll-related wait keys

__poll_t is also used as wait key in some waitqueues.
Verify that wait_..._poll() gets __poll_t as key and
provide a helper for wakeup functions to get back to
that __poll_t value.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

mm/memcontrol.c

@@ -3777,7 +3777,7 @@ static int memcg_event_wake(wait_queue_entry_t *wait, unsigned mode,
 	struct mem_cgroup_event *event =
 		container_of(wait, struct mem_cgroup_event, wait);
 	struct mem_cgroup *memcg = event->memcg;
-	unsigned long flags = (unsigned long)key;
+	__poll_t flags = key_to_poll(key);
 
 	if (flags & POLLHUP) {
 		/*