dm: record old_sector in dm_target_io before calling map function

Prep for being able to defer trace_block_bio_remap() until when the
bio is remapped and submitted by the DM target.

Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>