osd: fix pps calculation from pgid.ps() and pgid.pool()

osd: fix pps calculation from pgid.ps() and pgid.pool()

The final placement seed needs to factor in pool, but that can't be
fed into stable_mod or you get weird results (for example, 1.ff and
1.adff won't necessary map to the same thing because of the
stable_mod).  Add pool to the stable_mod result, instead.  The seed
itself doesn't need to be bounded; it's just an input for CRUSH.
Just so long as there are a limited number of such inputs for a given
pool.