TCP syn flood handling regression

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A typical port 80 SYN flood started up to one of our clusters, but this
time, it didn't work so well. Legitimate connections and trying to fetch
server-status via localhost would hang for ~30 seconds before responding,
even though though the box had plenty of spare cycles. An strace of all
Apache processes showed quite a bit of sleeping in accept4().

This was with 3.2.9, so I went back in kernel builds and found that 3.1
and 3.0 were also broken, while 2.6.39 works as I remember -- when syn
cookies are enabled, everything just works and is fast. The DoS kept up,
so I was able to feed a bit to a node to do some bisection.

Of course, the DoS stopped literally seconds before the last bisection
test, but I got it down to:

# good: [0e734419923bd8e599858f8fc196c7804bb85564] ipv4: Use inet_csk_route_child_sock() in DCCP and TCP.
# bad: [ea4fc0d6193ff56fcef39b0d2210d402a7acb5f0] ipv4: Don't use rt->rt_{src,dst} in ip_queue_xmit().

...leaving ea4fc0d6193ff56fcef39b0d2210d402a7acb5f0 and
d9d8da805dcb503ef8ee49918a94d49085060f23 as culprits.

I've stared at them but can't see what could be doing this.

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