__bio_clone: don't calculate hw/phys segment counts

If the users sets a new ->bi_bdev on the bio after __bio_clone() has
returned it, the "segment counts valid" flag still remains even though
it may be different with the new target. So don't calculate segment
counts in __bio_clone().

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>