io_uring: remove req init from io_get_req()

io_get_req() do two different things: io_kiocb allocation and
initialisation. Move init part out of it and rename into
io_alloc_req(). It's simpler this way and also have better data
locality.

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