io_uring: prep req when do IOSQE_ASYNC

Whenever IOSQE_ASYNC is set, requests will be punted to async without
getting into io_issue_req() and without proper preparation done (e.g.
io_req_defer_prep()). Hence they will be left uninitialised.

Prepare them before punting.

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