sheep: handle a case of small buffer in cinfo collection

Current cinfo_collection_work() and SD_OP_VDI_STATE_SNAPSHOT_CTL
doesn't handle a case of small buffer. This patch solves it.

Cc: Saeki Masaki <saeki.masaki@po.ntts.co.jp>
Cc: Yuka Kawasaki <kawasaki.yuka@po.ntts.co.jp>
Signed-off-by: Hitoshi Mitake <mitake.hitoshi@lab.ntt.co.jp>