sheep: correct locking of vdi status in inode coherence protocol

Currnet critical sections of vdi status in inode coherence protocl (in
functions validate_myself(), invalidate_other_nodes(),
inode_coherence_update()) are illegally short. This patch let the
the functions lock the vdi_state_lock correctly.

Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>