cls/rgw: preserve olh entry's name on last unlink

cls/rgw: preserve olh entry's name on last unlink

When rgw_bucket_unlink_instance removes the last instance of a name, it
also clears the value of rgw_bucket_olh_entry.key. However, bucket index
resharding uses this key when choosing its shard placement, so an empty
key causes all of these olh entries to be misplaced in shard 0. After
reshard, all of the olh recovery/cleanup logic would be sent to the
correct shard, and these misplaced olh entries would never be cleaned
up.

Preserving the key's name on last unlink allows the olh entry to be
resharded correctly and cleaned up normally.

Fixes: https://tracker.ceph.com/issues/46456
Signed-off-by: Casey Bodley <cbodley@redhat.com>