block: Refactor bio_clone_bioset() for immutable biovecs

bio_clone() needs to produce a bio that's suitable for the caller to
munge with the biovec. Part of the immutable biovec patch series is
fixing stuff up so that submitting partially completed bios is safe and
works: thus, we now need bio_clone() on a partially completed bio to
produce a bio for which bi_idx and bi_bvec done are 0 - like they would
be if the caller had just allocated a new bio.

Signed-off-by: Kent Overstreet <kmo@daterainc.com>
Cc: Jens Axboe <axboe@kernel.dk>