io_uring: assign non-fixed early for async work

We defer file assignment to ensure that fixed files work with links
between a direct accept/open and the links that follow it. But this has
the side effect that normal file assignment is then not complete by the
time that request submission has been done.

For deferred execution, if the file is a regular file, assign it when
we do the async prep anyway.

Signed-off-by: Jens Axboe <axboe@kernel.dk>