rados: fix typo

cct is a member of the ObjBencher parent that we are trying to
initialize; need to pass in cct_ here.

Broken in 5dd0a83f917cb73a9985c94b9b6c2309c66d0ce3.

Fixes: #6256
Signed-off-by: Sage Weil <sage@inktank.com>