rgw: don't use put_system_obj() for data objects

Use the new interface instead.

Signed-off-by: Yehuda Sadeh <yehuda@redhat.com>
rgw: get rid of put_obj_meta(), replace with put_system_obj()

Last use of put_obj_meta() was for system object, simplify it for that
specific use case and rename.

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