bcache: comment on direct access to bvec table

All direct access to bvec table are safe even after multipage bvec is
supported.

Cc: linux-bcache@vger.kernel.org
Acked-by: Coly Li <colyli@suse.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>