io_uring: fix double poll leak on repolling

We have re-polling for partial IO, so a request can be polled twice. If
it used two poll entries the first time then on the second
io_arm_poll_handler() it will find the old apoll entry and NULL
kmalloc()'ed second entry, i.e. apoll->double_poll, so leaking it.

Fixes: 10c8733 ("io_uring: allow re-poll if we made progress")
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Link: https://lore.kernel.org/r/fee2452494222ecc7f1f88c8fb659baef971414a.1655852245.git.asml.silence@gmail.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -7208,6 +7208,7 @@ static int io_arm_poll_handler(struct io_kiocb *req, unsigned issue_flags)
 		mask |= EPOLLEXCLUSIVE;
 	if (req->flags & REQ_F_POLLED) {
 		apoll = req->apoll;
+		kfree(apoll->double_poll);
 	} else if (!(issue_flags & IO_URING_F_UNLOCKED) &&
 		   !list_empty(&ctx->apoll_cache)) {
 		apoll = list_first_entry(&ctx->apoll_cache, struct async_poll,