io_uring: alloc req only after getting sqe

As io_get_sqe() split into 2 stage get/consume, get an sqe before
allocating io_kiocb, so no free_req*() for a failure case is needed,
and inline back __io_req_do_free(), which has only 1 user.

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