block: change rq_integrity_vec to respect the iterator

If we allocate a bio that is larger than NVMe maximum request size,
attach integrity metadata to it and send it to the NVMe subsystem, the
integrity metadata will be corrupted.

Splitting the bio works correctly. The function bio_split will clone the
bio, trim the iterator of the first bio and advance the iterator of the
second bio.

However, the function rq_integrity_vec has a bug - it returns the first
vector of the bio's metadata and completely disregards the metadata
iterator that was advanced when the bio was split. Thus, the second bio
uses the same metadata as the first bio and this leads to metadata
corruption.

This commit changes rq_integrity_vec, so that it calls mp_bvec_iter_bvec
instead of returning the first vector. mp_bvec_iter_bvec reads the
iterator and uses it to build a bvec for the current position in the
iterator.

The "queue_max_integrity_segments(rq->q) > 1" check was removed, because
the updated rq_integrity_vec function works correctly with multiple
segments.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Reviewed-by: Anuj Gupta <anuj20.g@samsung.com>
Reviewed-by: Kanchan Joshi <joshi.k@samsung.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Link: https://lore.kernel.org/r/49d1afaa-f934-6ed2-a678-e0d428c63a65@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>