Increment the route table gen count after a modify

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.

MFC after: 1 week
Sponsored by: Dell EMC Isilon
Differential Revision: https://reviews.freebsd.org/D13990
Reviewed by: karels