dm: fix multipath regression due to initializing wrong request

Commit febf7158 ("block: require blk_rq_prep_clone() be given an
initialized clone request") introduced a regression by calling
blk_rq_init() on the original request rather than the clone
request that is passed to setup_clone().

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Fixes: febf7158 ("block: require blk_rq_prep_clone() be given an initialized clone request")
Signed-off-by: Jens Axboe <axboe@fb.com>