crimson: avoid creating temporary with protected dtor

crimson: avoid creating temporary with protected dtor

to address the compiler error from clang 9.0.1:

../src/crimson/os/cyanstore/cyan_collection.cc:16:34: error: temporary of type 'boost::intrusive_ref_counter<Object, boost::thread_unsafe_counter>' has protected destructor
  return new crimson::os::Object{};
                                 ^
/opt/ceph/include/boost/smart_ptr/intrusive_ref_counter.hpp:157:30: note: declared protected here
    BOOST_DEFAULTED_FUNCTION(~intrusive_ref_counter(), {})
                             ^

Signed-off-by: Kefu Chai <kchai@redhat.com>