block: use bio_clone_fast() in blk_rq_prep_clone()
Request cloning clones bios in the request to track the completion
of each bio.
For that purpose, we can use bio_clone_fast() instead of bio_clone()
to avoid unnecessary allocation and copy of bvecs.
This patch reduces memory footprint of request-based device-mapper
(about 1-4KB for each request) and is a preparation for further
reduction of memory usage by removing unused bvec mempool.