Use the queue wide write back cache tracking insted of duplicating the
value in strut rq_wb.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/20231226090747.204969-1-hch@lst.de


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