blk-integrity: properly account for segments

Both types of merging when integrity data is used are miscounting the
segments:

Merging two requests wasn't accounting for the new segment count, so add
the "next" segment count to the first on a successful merge to ensure
this value is accurate.

Merging a bio into an existing request was double counting the bio's
segments, even if the merge failed later on. Move the segment accounting
to the end when the merge is successful.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Martin K. Petersen <martin.petersen@oracle.com>
Signed-off-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20240913182854.2445457-4-kbusch@meta.com


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