osd: enable HASHPSPOOL by default

Much like the CRUSH tunables, this first appears in kernel v3.9.

Unlike the CRUSH tunables, it does not appear in Ceph until v0.64
(post cuttlefish, pre dumpling).

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #930 from ceph/wip-hashpspool

enable hashpspool by default

Reviewed-by: Samuel Just <sam.just@inktank.com>