perf regression from ipv6: use net->rt_genid to check dst validity

Hi everyone,

I tracked down a perf regression last week in our 3.10-stable based
kernel.  fib6_lookup_1 was at the top of the profiles, being called very
frequently during sends.  The 2.6.38 kernel we were comparing
against only called fib6_lookup_1 during recv.

The call chain was doing the lookups because we were always tossing the
destination cache.  A little trial and error led me to this commit:

	commit 6f3118b571b8a4c06c7985dc3172c3526cb86253
	Author: Nicolas Dichtel [off-list ref]
	Date:   Mon Sep 10 22:09:46 2012 +0000

	    ipv6: use net->rt_genid to check dst validity

The workload was our in memory database, and dropping this commit gave
us a 10% boost to overall queries per second.

Moving up to mainline, it looks like we're still failing the validity
check most of the time.  A few printks show when we do fail, it's always
this line:

       if (rt->rt6i_genid != rt_genid_ipv6(dev_net(rt->dst.dev)))

The cached dst had a genid of 2 and the dev_net version was 3.

What I haven't done yet is fully reproduce the 10% hit on mainline.  I
have a few patches to port in and I'll get a workload running on 3.15.
But it doesn't look like this part has changed.  Somehow we're hanging
onto a destination cache with an old genid and we're hammering on lookups
because of it.

Any ideas before I shower things with printk?

-chris