io_uring: clean up io_req_free_batch_finish()

io_req_free_batch_finish() is final and does not permit struct req_batch
to be reused without re-init. To be more consistent don't clear ->task
there.

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