io_uring: drop the old style inflight file tracking

io_uring tracks requests that are referencing an io_uring descriptor to
be able to cancel without worrying about loops in the references. Since
we now assign the file at execution time, the easier approach is to drop
a potentially problematic reference before we punt the request. This
eliminates the need to special case these types of files beyond just
marking them as such, and simplifies cancelation quite a bit.

This also fixes a recent issue where an async punted tee operation would
with the io_uring descriptor as the output file would crash when
attempting to get a reference to the file from the io-wq worker. We
could have worked around that, but this is the much cleaner fix.

Fixes: 6bf9c47a ("io_uring: defer file assignment")
Reported-by:  <syzbot+c4b9303500a21750b250@syzkaller.appspotmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>