block: fix mismerge in bvec_advance

When Jens merged my commit to only allow contiguous page structs in a
bio_vec with Ming's 5.1 fix to ensue the bvec length didn't overflow
we failed to keep the removal of the expensive nth_page calls.  This
commits adds them back as intended.

Fixes: 5c61ee2c ("Merge tag 'v5.1-rc6' into for-5.2/block")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>