rtsock: always set m_pkthdr.rcvif when queueing on netisr

netisr uses global workstreams and after dequeueing an mbuf it
uses rcvif to get the VNET of the mbuf. Of course, this is not
needed when kernel is compiled without VIMAGE. It came out that
routing socket does not set rcvif if compiled without VIMAGE.
Make this assignment not depending on VIMAGE option.

Fixes: 6871de9363e5