fix unbalanced page refcounting in bio_map_user_iov

bio_map_user_iov and bio_unmap_user do unbalanced pages refcounting if
IO vector has small consecutive buffers belonging to the same page.
bio_add_pc_page merges them into one, but the page reference is never
dropped.

Cc: stable@vger.kernel.org
Signed-off-by: Vitaly Mayatskikh <v.mayatskih@gmail.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
int bio_associate_blkcg(struct bio *bio, struct cgroup_subsys_state *blkcg_css)
{
	if (unlikely(bio->bi_css))
		return -EBUSY;
	css_get(blkcg_css);
	bio->bi_css = blkcg_css;
	return 0;
}

int bio_associate_blkcg(struct bio *bio, struct cgroup_subsys_state *blkcg_css)
{
	if (unlikely(bio->bi_css))
		return -EBUSY;
	css_get(blkcg_css);
	bio->bi_css = blkcg_css;
	return 0;
}
