osd: do not make pg_pool_t incompat when hit_sets are enabled

If we enable HitSet tracking, the OSD needs to know this, but clients do not
care.  Setting the compat version is too heavyweight as it locks out older
kernels (*any* currents, currently) who are unaffected by the new fields.

Signed-off-by: Sage Weil <sage@inktank.com>