CrushWrapper: pick a ruleset same as rule_id

Originally in the add_simple_ruleset funtion, the ruleset_id
is not reused but rule_id is reused. So after some add/remove
against rules, the newly created rule likely to have
ruleset!=rule_id.

We dont want this happen because we are trying to hold the constraint
that ruleset == rule_id.

This patch allow ruleset_id be reused. After having this patch,
add/remove rules will not break the constraint.

Signed-off-by: Xiaoxi Chen <xiaoxi.chen@intel.com>
you may misread the line:) , actually it's a bug and I happened to find it during run the test so I fixed it, and since it only one line ,so I don't want a separate pool request.

without making sure the rule exists, how can we access the mask of the rule? 
so without such change, the code may try to access a non-existing rule's mask, which is likely to cause segment fault.  you can simply trigger this bug by:
1. add a new rule (assume rule id=1 and ruleset = 1)
2. add a new rule( assume rule id = 2 and ruleset=2)
3. delete the rule with id=1
4. set a pool with ruleset =1 (it will call ruleset_exists function, and hit this bug)