cls/rgw: non-versioned listings skip past version suffix

cls/rgw: non-versioned listings skip past version suffix

when skipping a versioned entry for a non-versioned listing, we must
advance the marker or risk infinite loops. in particular, plain entries
converted by convert_plain_entry_to_versioned() sort at the end of an
object's versions, but have an empty version id whose retry would start
back at the beginning of the object's versions

Fixes: https://tracker.ceph.com/issues/70399
Signed-off-by: Casey Bodley <cbodley@redhat.com>