SQUASH "OSD: add a RWLock pg_map_lock"

We're repurposing the _lookup_lock_pg_with_map_lock_held() function
to refer to the pg_map_lock. handle_pg_query and handle_pg_remove switch
to use that version, because they're holding pg_map_lock already and
we know the PG they're referring to exists.

Signed-off-by: Greg Farnum <greg@inktank.com>