rbd: Refactor bio cloning, don't clone biovecs

rbd: Refactor bio cloning, don't clone biovecs

Now that we've got drivers converted to the new immutable bvec
primitives, bio splitting becomes much easier. In a few patches,
bio_clone() will be changed to share the old bio's bvec instead of
copying it, and bio_split() will do exactly what's being done 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
Signed-off-by: Jens Axboe <axboe@kernel.dk>