sheep: use correct number of copies during reference decrement

In a case of nr_zones < nr_copies of deleted VDI, current sheep panics
because it cannot obtain correct list of nodes which hold target
ledgers. This patch solves the problem.

Reported-by: Valerio Pachera <sirio81@gmail.com>
Tested-by: Valerio Pachera <sirio81@gmail.com>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>