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.
this is the current one.  the 17 above should be changed to 18, and the new field added at the end of the encoded structure.