ReplicatedPG: send entire stats on OP_BACKFILL_FINISH

Otherwise, we update the stat.stat structure, but not the
stat.invalid_stats part.  This will result in a recently
split primary propogating the invalid stats but not the
invalid marker.  Sending the whole pg_stat_t structure
also mirrors MOSDSubOp.

Fixes: #4557
Backport: bobtail
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>