Increment the route table gen count after a modify

[rstone]

Summary

Increment the route table generation count after modifying a
route. This signals back to TCP connections that they need to
update their L2 caches as the gateway for their route may have
changed. This is a heavier hammer than is needed, strictly
speaking, but route changes will be unlikely enough that the
performance effects of invalidating all connection route caches
should be negligible.

[• karels]

Why is this necessary, or desirable? If a route that is cached is modified, that doesn't invalidate it. It should still be the best route if it was before.

[• karels]

I just realized that this is necessary when the L3 gateway changes. The L3 cache doesn't need to be invalidated, but L2 does. A comment to this effect would be helpful.

[rstone]

Add comment explaining generation increment

[• karels]

Looks good.