rgw: fix error handling in ListBucketIndexesCR

[cbodley]

the call to set_state() returns 0, when we want operate() to return the
error code instead. use set_cr_error() to do this

Fixes: http://tracker.ceph.com/issues/21735

[cbodley]

to test, i used this diff to inject an error in RGWListBucketIndexesCR::operate():

   int operate() override {
     reenter(this) {
+#if 0
       yield {
         string entrypoint = string("/admin/metadata/bucket.instance");
         /* FIXME: need a better scaling solution here, requires streaming output */
         call(new RGWReadRESTResourceCR<list<string> >(store->ctx(), sync_env->conn, sync_env->http_manager,
                                                       entrypoint, NULL, &result));
       }
+#else
+      set_retcode(-EINVAL); // inject failure
+#endif

without the fix applied, data sync status got stuck on full sync:

 data sync source: 6e5c3ecc-50ea-4a1f-a6b1-ecfbbb2bd259 (na-1)
                   syncing
                   full sync: 128/128 shards
                   full sync: 0 buckets to sync
                   incremental sync: 0/128 shards
                   data is behind on 128 shards

with the fix applied, data sync status instead gets stuck in the correct 'preparing for full sync' state:

 data sync source: f5628297-2ae2-4a62-a02a-771737830af8 (na-1)
                   preparing for full sync
                   full sync: 1/1 shards
                   full sync: 0 buckets to sync
                   incremental sync: 0/1 shards
                   data is behind on 1 shards

with the change to RGWDataSyncControlCR, these errors are retried with backoff instead of returning control and stopping the data sync thread

[yuriw]

wip-yuri-testing-2017-10-10-2113

[yuriw]

removed tags per @cbodley see test results above

[yuriw]

wip-yuri4-testing-2017-10-23-2051