io_uring: dismantle req early and remove need_iter

Every request in io_req_multi_free() is has ->file set. Instead of
pointlessly defering and counting reqs with file, dismantle it on place
and save for batch dealloc.

It also saves us from potentially skipping io_cleanup_req(), put_task(),
etc. Never happens though, becacuse ->file is always there.

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