Change CrushWrapper::crush to private

Currently in CrushWrapper, the member "struct crush_map *crush"  is a public member,
so people can break the encapsulation and manipulate directly to the crush structure.

This is not a good practice for encapsulation and will lead to inconsistent if code
mix use the CrushWrapper API and crush C API.A simple example could be:
1.some code use crush_add_rule(C-API) to add a rule, which will not set the have_rmap flag to false in CrushWrapper
2.another code using CrushWrapper trying to look up the newly added rule by name will get a -ENOENT.

This patch move CrushWrapper::crush to private, together with three reverse map(type_rmap, name_rmap, rule_name_rmap)
and also change codes accessing the CrushWrapper::crush to make it compile.

Signed-off-by: Xiaoxi Chen <xiaoxi.chen@intel.com>
we're assering ret == 0 each time, but ret isn't getting reassigned on each set_rule_step call ...