Reduce code duplication for inpcb route caching

[rstone]

Summary

Add a new macro to clear both the L3 and L2 route caches, to
hopefully prevent future instances where only the L3 cache was
cleared when both should have been.

[vangyzen]

sys/net/route.h

419

I generally prefer inline functions over macros, but this is fine.

sys/netinet/ip_output.c

305–306

This drops the assignment of NULL to the local rte.

[rstone]

NULL-out rte after route cache is invalidated

[• karels]

Was there a place where only L3 was invalidated?

In any case, this seems like a good change.