dm: prepare for allocating blk-mq clone requests in target

For blk-mq request-based DM the responsibility of allocating a cloned
request will be transfered from DM core to the target type.

To prepare for conditionally using this new model the original
request's 'special' now points to the dm_rq_target_io because the
clone is allocated later in the block layer rather than in DM core.
Signed-off-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>