runsc: copy gateway from the pod network interface.
runsc: copy gateway from the pod network interface.

PiperOrigin-RevId: 205334841
Change-Id: Ia60d486f9aae70182fdc4af50cf7c915986126d7

google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor,google/gvisor