bio-integrity: Restore original iterator on verify stage

Currently ->verify_fn not woks at all because at the moment it is called
bio->bi_iter.bi_size == 0, so we do not iterate integrity bvecs at all.

In order to perform verification we need to know original data vector,
with new bvec rewind API this is trivial.

testcase: https://github.com/dmonakhov/xfstests/commit/3c6509eaa83b9c17cd0bc95d73fcdd76e1c54a85



Reviewed-by: Hannes Reinecke <hare@suse.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Dmitry Monakhov <dmonakhov@openvz.org>
[hch: adopted for new status values]
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>