mds: handle bad purge queue item encoding

mds: handle bad purge queue item encoding

The bad encoding was introduced by commit a88f8d5eb4

Signed-off-by: "Yan, Zheng" <zyan@redhat.com>
(cherry picked from commit ba06fcbe3e8345dc2c4c4c3dc3bcc18acf5ab076)
Conflicts:
        src/mds/PurgeQueue.cc: advance changed to +=
Fixes: https://tracker.ceph.com/issues/36635
Note: This commit from v13.2.3 fixes a bad backport in v13.2.2. It is
      also required in Octopus/Nautilus to handle upgrades.
(cherry picked from commit b73d1989bcbea227017607f8dd6e79633ec11f8f)
Conflicts:
            src/mds/PurgeQueue.cc: += changed to advance