multisite: bi_list() decode failures
Steps to reproduce:
1) Set up a multisite configuration with two zones.
2) Create a bucket and upload some objects on the master zone.
3) Run `radosgw-admin bi list --bucket=foo` on the master zone. Fails with "ERROR: bi_list(): (5) Input/output error"
During bi_list(), the helper function list_plain_entries() is pulling in rgw_bi_log_entry entries as well. When it tries to decode them as type rgw_bucket_dir_entry, it fails with -EIO.