rgw/admin: 'bucket stats' displays non-empty mtime

[cbodley]

fixes two regressions for https://tracker.ceph.com/issues/58932:

• 90aec61 rgw_get_system_obj() stopped getting mtimes from cache/librados
• 72d1a36 bucket_stats() left mtime uninitialized

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[cbodley]

~/ceph/build $ s3cmd mb s3://testbucket                                                                                        
Bucket 's3://testbucket/' created

~/ceph/build $ bin/radosgw-admin bucket stats --bucket testbucket
2023-03-07T18:12:59.353-0500 7fedf5432a00 -1 WARNING: all dangerous and experimental features are enabled.
2023-03-07T18:12:59.360-0500 7fedf5432a00 -1 WARNING: all dangerous and experimental features are enabled.
2023-03-07T18:12:59.361-0500 7fedf5432a00 -1 WARNING: all dangerous and experimental features are enabled.
{
    "bucket": "testbucket",
    "num_shards": 11,
    "tenant": "",
    "zonegroup": "7f6d07c1-c5f5-4dd7-a897-cb52fa74bcff",
    "placement_rule": "default-placement",
    "explicit_placement": {
        "data_pool": "",
        "data_extra_pool": "",
        "index_pool": ""
    },
    "id": "f414a934-f454-40be-b58e-ab70f6e13f27.4147.1",
    "marker": "f414a934-f454-40be-b58e-ab70f6e13f27.4147.1",
    "index_type": "Normal",
    "owner": "testid",
    "ver": "0#1,1#1,2#1,3#1,4#1,5#1,6#1,7#1,8#1,9#1,10#1",
    "master_ver": "0#0,1#0,2#0,3#0,4#0,5#0,6#0,7#0,8#0,9#0,10#0",
    "mtime": "2023-03-07T23:12:50.027215Z",
    "creation_time": "2023-03-07T23:12:47.904675Z",
    "max_marker": "0#,1#,2#,3#,4#,5#,6#,7#,8#,9#,10#",
    "usage": {},
    "bucket_quota": {
        "enabled": false,
        "check_on_raw": false,
        "max_size": -1,
        "max_size_kb": 0,
        "max_objects": -1
    }
}

[cbodley]

passed qa in https://pulpito.ceph.com/cbodley-2023-03-10_19:16:19-rgw-wip-cbodley-testing-distro-default-smithi/