aio: only return events requested in poll_mask() for IOCB_CMD_POLL

The ->poll_mask() operation has a mask of events that the caller
is interested in, but not all implementations might take it into
account.  Mask the return value to only the requested events,
similar to what the poll and epoll code does.

Reported-by: Avi Kivity <avi@scylladb.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/aio.c

@@ -1661,7 +1661,7 @@ static int aio_poll_wake(struct wait_queue_entry *wait, unsigned mode, int sync,
 	if (mask && !(mask & req->events))
 		return 0;
 
-	mask = file->f_op->poll_mask(file, req->events);
+	mask = file->f_op->poll_mask(file, req->events) & req->events;
 	if (!mask)
 		return 0;
 
@@ -1719,7 +1719,7 @@ static ssize_t aio_poll(struct aio_kiocb *aiocb, struct iocb *iocb)
 
 	spin_lock_irq(&ctx->ctx_lock);
 	spin_lock(&req->head->lock);
-	mask = req->file->f_op->poll_mask(req->file, req->events);
+	mask = req->file->f_op->poll_mask(req->file, req->events) & req->events;
 	if (!mask) {
 		__add_wait_queue(req->head, &req->wait);
 		list_add_tail(&aiocb->ki_list, &ctx->active_reqs);