loop: simplify do_req_filebacked()

loop: simplify do_req_filebacked()

lo_rw_aio() is only called for READ/WRITE operation, which can be
figured out from request directly, so remove 'rw' parameter from
lo_rw_aio(), meantime rename the local variable as 'dir' which makes
the check more readable in lo_rw_aio().

Meantime add lo_rw_simple() so that do_req_filebacked() can be
simplified in the following way:

```
	if (cmd->use_aio)
		return lo_rw_aio(lo, cmd, pos);
	else
		return lo_rw_simple(lo, rq, pos);
```

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250322012617.354222-2-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>