block: Don't save/copy bvec array anymore

Now that drivers have been converted to the bvec_iter primitives, they
shouldn't be modifying the biovec anymore and thus saving it is
unnecessary - code that was previously making a backup of the bvec array
can now just save bio->bi_iter.

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