dm: use bioset_init_from_src() to copy bio_set

We can't just copy and clear a bio_set, use the bio helper to
setup a new bio_set with the settings from another one.

Fixes: 6f1c819c ("dm: convert to bioset_init()/mempool_init()")
Reported-by: Venkat R.B <vrbagal1@linux.vnet.ibm.com>
Tested-by: Venkat R.B <vrbagal1@linux.vnet.ibm.com>
Tested-by: Li Wang <liwang@redhat.com>
Reviewed-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>