Merge branch 'tcp-add-three-static-keys'

Eric Dumazet says:

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tcp: add three static keys

Recent addition of per TCP socket rx/tx cache brought
regressions for some workloads, as reported by Feng Tang.

It seems better to make them opt-in, before we adopt better
heuristics.

The last patch adds high_order_alloc_disable sysctl
to ask TCP sendmsg() to exclusively use order-0 allocations,
as mm layer has specific optimizations.
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Signed-off-by: David S. Miller <davem@davemloft.net>