[RFC] tcp: use order-3 pages in tcp_sendmsg()

[Eric Dumazet <eric.dumazet@gmail.com>]

We currently use per socket page reserve for tcp_sendmsg() operations.

Its done to raise the probability of coalescing small write() in to
single segments in the skbs.

But it wastes a lot of memory for applications handling a lot of mostly
idle sockets, since each socket holds one page in sk->sk_sndmsg_page

I did a small experiment to use order-3 pages and it gave me a 10% boost
of performance, because each TSO skb can use only two frags of 32KB,
instead of 16 frags of 4KB, so we spend less time in ndo_start_xmit() to
setup the tx descriptor and TX completion path to unmap the frags and
free them.

We also spend less time in tcp_sendmsg(), because we call page allocator
8x less often.

Now back to the per socket page, what about trying to factorize it ?

Since we can sleep (or/and do a cpu migration) in tcp_sendmsg(), we cant
really use a percpu page reserve as we do in __netdev_alloc_frag()

We could instead use a per thread reserve, at the cost of adding a test
in task exit handler.

Recap :

1) Use a per thread page reserve instead of a per socket one
2) Use order-3 pages (or order-0 pages if page size is >= 32768)