dog/vdi: fix bug in dog vdi check

If condition is never satisfied since nr_copies
is the minimum of inode->nr_copies and sd_zones_nr.

Signed-off-by: Bingpeng Zhu <bingpeng.zbp@alibaba-inc.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>