cls/rgw: list_plain_entries() stops before bi_log entries

cls/rgw: list_plain_entries() stops before bi_log entries

list_plain_entries() was using encode_obj_versioned_data_key() to set
its end_key, which gives a prefix of BI_BUCKET_OBJ_INSTANCE_INDEX[=2]

that range between start_key and end_key would not only span the
BI_BUCKET_OBJS_INDEX[=0] prefixes, but BI_BUCKET_LOG_INDEX[=1] prefixes
as well. this can result in list_plain_entries() trying and failing to
decode a rgw_bi_log_entry as a rgw_bucket_dir_entry

Fixes: http://tracker.ceph.com/issues/19876
Signed-off-by: Casey Bodley <cbodley@redhat.com>