io_uring: use original request task for inflight tracking

In prior kernels, we did file assignment always at prep time. This meant
that req->task == current. But after deferring that assignment and then
pushing the inflight tracking back in, we've got the inflight tracking
using current when it should in fact now be using req->task.

Fixup that error introduced by adding the inflight tracking back after
file assignments got modifed.

Fixes: 9cae36a094e7 ("io_uring: reinstate the inflight tracking")
Signed-off-by: Jens Axboe <axboe@kernel.dk>
