block: remove 'req->part' check for stats accounting

If RQF_IO_STAT is set, then accounting is enabled. There's no need to
further gate this on req->part being set or not, RQF_IO_STAT should
never be set if accounting is not being done for this request.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>