bcache: avoid to use bio_for_each_segment_all() in bch_bio_alloc_pages()

bch_bio_alloc_pages() is always called on one new bio, so it is safe
to access the bvec table directly. Given it is the only kind of this
case, open code the bvec table access since bio_for_each_segment_all()
will be changed to support for iterating over multipage bvec.

Acked-by: Coly Li <colyli@suse.de>
Reviewed-by: Omar Sandoval <osandov@fb.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

drivers/md/bcache/util.c

@@ -270,7 +270,11 @@ int bch_bio_alloc_pages(struct bio *bio, gfp_t gfp_mask)
 	int i;
 	struct bio_vec *bv;
 
-	bio_for_each_segment_all(bv, bio, i) {
+	/*
+	 * This is called on freshly new bio, so it is safe to access the
+	 * bvec table directly.
+	 */
+	for (i = 0, bv = bio->bi_io_vec; i < bio->bi_vcnt; bv++, i++) {
 		bv->bv_page = alloc_page(gfp_mask);
 		if (!bv->bv_page) {
 			while (--bv >= bio->bi_io_vec)