kernel-pfroute: Maintain virtual flag when repopulating interface addrs

kernel-pfroute: Maintain virtual flag when repopulating interface addrs

When adding a virtual IP on a TUN interface, the interface might get
activated (in terms of receiving the event) after we've already set the
virtual flag for the added address.  As the activation repopulates the
addresses on the interface, this cleared the flag and the address would
no longer be treated as virtual IP when installing routes for CHILD_SAs
that reference it in their local traffic selectors.

Closes strongswan/strongswan#1807