dm: move cloning the bio into alloc_tio

Move the call to __bio_clone_fast and the assignment of ->len_ptr from
the callers into alloc_tio to prepare for changes to the bio clone API.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Link: https://lore.kernel.org/r/20220202160109.108149-6-hch@lst.de


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