blk-mq: avoid a mess of casts for blk_end_sync_rq

Instead of trying to cast a __bitwise 32-bit integer to a larger integer
and then a pointer, just allow a struct with the blk_status_t and the
completion on stack and set the end_io_data to that.  Use the
opportunity to move the code to where it belongs and drop rather
confusing comments.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>
Link: https://lore.kernel.org/r/20220524121530.943123-3-hch@lst.de


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