ceph: fix get_oldest_context()

For no snapshot case, we should use ci->truncate_{seq,size}.

Fixes: 5f743e45 ("ceph: record truncate size/seq for snap data writeback")
Signed-off-by: Geng, Jichao <geng.jichao@h3c.com>
Signed-off-by: Yan, Zheng <zyan@redhat.com>