PG: constify the init() function params

Signed-off-by: Greg Farnum <greg@inktank.com>
OSD: Juggle the locking when resurrecting a PG

Don't hold the old PG's lock in _create_lock_pg. Instead, just copy the
necessary data bits into a holding location. Note that this means we aren't
protecting it against change while the new PG is created, which I *think*
is okay...

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

hunting #7891