Explicitly change the link state when we assingn an address.

Since we are setting IFF_UP flag on SIOCSIFADDR, it is possible, that
after this link state information still not initialized properly.
This leads to problems with routing, since now interface has
IFCAP_LINKSTATE capability and a route is considered as working only
when interface's link state is in LINK_STATE_UP (see RT_LINK_IS_UP()
macro).

Reported by: Marek Zarychta
MFC after: 3 days