crush: add a new flag HASHPSPOOL2 for the congruential pps algorithm

We did some optimization for the issue of unbalanced pg distribution among osds in previous work. Please refer to <a href="https://github.com/ceph/ceph/pull/2402">#2402</a> for details.

In this commit, we updated codes for adding a new hashpspool flag HASHPSPOOL2 for the new congruential pps hash algorithm as per previous comments.