rbd: Refactor bio cloning

Now that we've got drivers converted to the new immutable bvec
primitives, bio splitting becomes much easier - this is how the new
bio_split() will work. (Someone more familiar with the ceph code could
probably use bio_clone_fast() instead of bio_clone() here).

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Yehuda Sadeh <yehuda@inktank.com>
Cc: Alex Elder <elder@inktank.com>
Cc: ceph-devel@vger.kernel.org