osd: use per-pool temp poolid for temp objects

osd: use per-pool temp poolid for temp objects

Previously, all temp objects had poolid == -1.  Instead, use -2 - poolid.
This, when combined with the PG hash, provides a unique temp namespace
per pg.

This has no impact on upgrade, since we delete all temp objects on startup
by collection (coll_t::is_temp()).

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