io_uring: reinstate the inflight tracking

After some debugging, it was realized that we really do still need the
old inflight tracking for any file type that has io_uring_fops assigned.
If we don't, then trivial circular references will mean that we never get
the ctx cleaned up and hence it'll leak.

Just bring back the inflight tracking, which then also means we can
eliminate the conditional dropping of the file when task_work is queued.

Fixes: d5361233

 ("io_uring: drop the old style inflight file tracking")
Signed-off-by: Jens Axboe <axboe@kernel.dk>