vsl: Fixed do_io unmapping request.

As explained above 'blk_rq_map_user' - mapping requests which, for
some reason (e.g. alignment of memory wrt. backing device) are
bounced when processed will refer to a different bio after
processing. Thus the caller needs to retain a reference to the old
bio (rq->bio after calling blk_rq_map_user) to pass when unmapping
the memory once the request finishes.
vsl: Thorough removal of cruft and debugging code from KV module.