vxlan: T6401: Avoid calling get_vxlan_vni_filter() unless we need it

bridge vni show dev vxlanX will exit with an error if no VNI filters
are installed, but the getter is used even when we haven't installed any.

This fix avoids fetching a list of VNI filters unless we know we've
created some.

(cherry picked from commit ac7ee2b36df23c3a4dd2be393132631556b6ef40)