io_uring: defer splice/tee file validity check until command issue

In preparation for not using the file at prep time, defer checking if this
file refers to a valid io_uring instance until issue time.

This also means we can get rid of the cleanup flag for splice and tee.

Cc: stable@vger.kernel.org # v5.15+
Signed-off-by: Jens Axboe <axboe@kernel.dk>