sheep/md: leave cluster if disk number equals zero in handling EIO

When a node has single disk and faces EIO, md.nr_disks will change
from 1 to 0. In such a case, md_do_recover() will not call
kick_recover(). This may lead to a problematic behavior. We should
call leave_cluster() if md.nr_disks equals 0.

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