when non-default striping is used, internal methods can operate on extra objects

Fixes: #9595

Signed-off-by: xinxin shu <xinxin.shu@intel.com>
add unittest for Striper::get_num_objects

Signed-off-by: xinxin shu <xinxin.shu@intel.com>
Merge pull request #2588 from xinxinsh/wip-9595

Fix #9595

Reviewed-by: Josh Durgin <josh.durgin@inktank.com>