sheep: invalidate inode status of other nodes before actual write

Current iscsi coherence protocol can cause inconsistency when a
gateway sheep process dies between inode update and invalidation.

This patch makes sure that inode update must cause invalidation even
if sheep dies before actual inode.

Tested-by: YAMADA Hideki <yamada.hideki@lab.ntt.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>