rt_setgate performs a series of tweaks to an rtable and the routes in

rt_setgate performs a series of tweaks to an rtable and the routes in
the rtable which should be serialised to ensure they're consistent.
unfortunately, rt_setgate is called from the network stack while it's
only holding shared NET_LOCK.

this uses the [X] protections as described in route.h to serialise the
changes, and reworks the code to try and keep enough stuff linked up
properly during the changes that it will still work if another cpu is
still using the rtentry structs while they still have shared net lock.

tested by and ok bluhm@