blk: Fix bio_io_vec index when checking bvec gaps

Corrects a coding error from earlier patch.

Reported by: Sagi Grimberg <sagig@dev.mellanox.co.il>
Signed-off-by: Keith Busch <keith.busch@intel.com>
Fixes: 03100aad ("block: Replace SG_GAPS with new queue limits mask")
Signed-off-by: Jens Axboe <axboe@fb.com>