rgw: fix error_repo segfault in data sync

[cbodley]

RGWDataSyncShardCR will only allocate an error_repo if it's doing
incremental sync, so RGWDataSyncSingleEntryCR needs to guard against a
null error_repo

also, RGWDataSyncShardCR::stop_spawned_services() was dropping the last
reference to the error_repo before calling drain_all(), which meant that
RGWDataSyncSingleEntryCR could still be holding a pointer

Fixes: http://tracker.ceph.com/issues/16603

Signed-off-by: Casey Bodley cbodley@redhat.com

[yehudasa]

@cbodley need to make sure that we aren't implicitly cross holding a reference (check that error_repo is not holding a reference to the caller via the cr infrastructure), otherwise neither error_repo is going to be freed, or RGWDataSyncSingleEntryCR

[cbodley]

@yehudasa i see. probably easier for RGWDataSyncShardCR to hold the error_repo reference until it destructs, i.e.:

   void stop_spawned_services() {
     lease_cr->go_down();
     if (error_repo) {
       error_repo->finish();
-      error_repo->put();
-      error_repo = NULL;
     }
   }

[yehudasa]

@cbodley lgtm