sheep: release object cache when vdi is released

Signed-off-by: Liu Yuan <tailai.ly@taobao.com>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>
util: remove the assertion in uatomic_set_false()

Current uatomic_set_false() has an assertion which hopes the passed
boolean variable is true. But this cannot be hold on any boolean
variables. If callers of this function want this assertion, they
should prepare their own wrappers.

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