ipv4: check rt_genid in dst_check

ipv4: check rt_genid in dst_check

commit d11a4dc18bf41719c9f0d7ed494d295dd2973b92
Author: Timo Teräs <timo.teras@iki.fi>
Date:   Thu Mar 18 23:20:20 2010 +0000

    ipv4: check rt_genid in dst_check

    Xfrm_dst keeps a reference to ipv4 rtable entries on each
    cached bundle. The only way to renew xfrm_dst when the underlying
    route has changed, is to implement dst_check for this. This is
    what ipv6 side does too.

    The problems started after 87c1e12b5eeb7b30b4b41291bef8e0b41fc3dde9
    ("ipsec: Fix bogus bundle flowi") which fixed a bug causing xfrm_dst
    to not get reused, until that all lookups always generated new
    xfrm_dst with new route reference and path mtu worked. But after the
    fix, the old routes started to get reused even after they were expired
    causing pmtu to break (well it would occationally work if the rtable
    gc had run recently and marked the route obsolete causing dst_check to
    get called).

    Signed-off-by: Timo Teras <timo.teras@iki.fi>
    Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is based on the above, with the addition of verifying blackhole
routes in the same manner.

Fixing the issue with blackhole routes as it was accomplished in mainline
would require pulling in a lot more code, and people were not interested
in pulling in all of the dependencies given the much higher risk of trying
to select the right subset of changes to include.  The addition of the
single line of "dst->obsolete = -1;" in ipv4_dst_blackhole() was much
easier to verify, and is in the spirit of the patch in question.
This is the minimal set of changes to fix the bug in question.

A test case is available here :
  http://marc.info/?l=linux-netdev&m=135015076708950&w=2

Signed-off-by: Benjamin LaHaise <bcrl@kvack.org>
Signed-off-by: Willy Tarreau <w@1wt.eu>