nvme-core: make implicit seed truncation explicit

The nvme_user_io.slba field is 64 bits wide. That value is copied into the
32-bit bio_integrity_payload.bip_iter.bi_sector field. Make that truncation
explicit to avoid that Coverity complains about implicit truncation. See
also Coverity ID 1056486 on http://scan.coverity.com/projects/linux.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Reviewed-by: Keith Busch <keith.busch@intel.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>