io_uring: don't delay io_init_req() error check

Don't postpone io_init_req() error checks and do that right after
calling it. There is no control-flow statements or dependencies with
sqe/submitted accounting, so do those earlier, that makes the code flow
a bit more natural.

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