block: copy back bounce buffer to user-space correctly in case of split

Copy back the bounce buffer to user-space in entirety when the parent
bio completes. The existing code uses bip_iter.bi_size for sizing the
copy, which can be modified. So move away from that and fetch it from
the vector passed to the block layer. While at it, switch to using
better variable names.

Fixes: 492c5d45 ("block: bio-integrity: directly map user buffers")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Anuj Gupta <anuj20.g@samsung.com>
Reviewed-by: Keith Busch <kbusch@kernel.org>
Link: https://lore.kernel.org/r/20241128112240.8867-3-anuj20.g@samsung.com


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