sheep: push dirty bits correctly for successive FLUSH req

This will fix a bug that 'qemu-img convert' fail to convert images intact with
default unsafe mode.

Signed-off-by: Liu Yuan <namei.unix@gmail.com>
Reviewed-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>
Signed-off-by: MORITA Kazutaka <morita.kazutaka@lab.ntt.co.jp>