[PATCH 0/2] Fix nh_saddr caching.

Julian, the following two patches should take all of the
issues we discovered the other week wrt. nh_saddr caching.

First, we use a generation ID to invalidate cache nh_saddr
settings any time a device goes up or an IP address is
added/removed.

Second, we move fa_scope up to fib_info->fib_scope, so that
we always use the correct scope when computing the cached
nh_saddr.

Please review this carefully as this is delicate code.

Thanks!