io_uring: fix a use after free in io_async_task_func()

The "apoll" variable is freed and then used on the next line.  We need
to move the free down a few lines.

Fixes: 0be0b0e3 ("io_uring: simplify io_async_task_func()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>