io_uring: missed req_init_async() for IOSQE_ASYNC

IOSQE_ASYNC branch of io_queue_sqe() is another place where an
unitialised req->work can be accessed (i.e. prior io_req_init_async()).
Nothing really bad though, it just looses IO_WQ_WORK_CONCURRENT flag.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>