block: reset bi_iter.bi_done after splitting bio

After the bio has been updated to represent the remaining sectors, reset
bi_done so bio_rewind_iter() does not rewind further than it should.

This resolves a bio_integrity_process() failure on reads where the
original request was split.

Fixes: 63573e35

 ("bio-integrity: Restore original iterator on verify stage")
Signed-off-by: Greg Edwards <gedwards@ddn.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>