cls_rgw: skip namespaced entries when listing bucket

cls_rgw: skip namespaced entries when listing bucket

We added a namespace to the entries, and sadly there can be
valid entries beyond it. Previously we assumed that once we
encounter the namespace tag we're done. That was wrong. We
need to skip the namespace and continue the listing from
there.

Signed-off-by: Yehuda Sadeh <yehuda@inktank.com>