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: NChristoph Hellwig <hch@lst.de>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

include/linux/bvec.h

@@ -133,11 +133,6 @@ static inline struct bio_vec *bvec_init_iter_all(struct bvec_iter_all *iter_all)
 	return &iter_all->bv;
 }
 
-static inline struct page *bvec_nth_page(struct page *page, int idx)
-{
-	return idx == 0 ? page : nth_page(page, idx);
-}
-
 static inline void bvec_advance(const struct bio_vec *bvec,
 				struct bvec_iter_all *iter_all)
 {
@@ -147,8 +142,7 @@ static inline void bvec_advance(const struct bio_vec *bvec,
 		bv->bv_page++;
 		bv->bv_offset = 0;
 	} else {
-		bv->bv_page = bvec_nth_page(bvec->bv_page, bvec->bv_offset /
-					    PAGE_SIZE);
+		bv->bv_page = bvec->bv_page + (bvec->bv_offset >> PAGE_SHIFT);
 		bv->bv_offset = bvec->bv_offset & ~PAGE_MASK;
 	}
 	bv->bv_len = min_t(unsigned int, PAGE_SIZE - bv->bv_offset,