block: Convert drivers to immutable biovecs

block: Convert drivers to immutable biovecs

Now that we've got a mechanism for immutable biovecs -
bi_iter.bi_bvec_done - we need to convert drivers to use primitives that
respect it instead of using the bvec array directly.

Signed-off-by: Kent Overstreet <[email protected]>
Cc: Jens Axboe <[email protected]>
Cc: NeilBrown <[email protected]>
Cc: Alasdair Kergon <[email protected]>
Cc: [email protected]