blk-wbt: move private information from blk-wbt.h to blk-wbt.c

A large part of blk-wbt.h is only used in blk-wbt.c, so move it there.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20230203150400.3199230-11-hch@lst.de


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