tcp: add a sysctl to config the tcp_default_init_rwnd

The default initial rwnd is hardcoded to 10.

Now we allow it to be controlled via
  /proc/sys/net/ipv4/tcp_default_init_rwnd
which limits the values from 3 to 100

This is somewhat needed because ipv6 routes are
autoconfigured by the kernel.

See "An Argument for Increasing TCP's Initial Congestion Window"
in https://developers.google.com/speed/articles/tcp_initcwnd_paper.pdf

Change-Id: I386b2a9d62de0ebe05c1ebe1b4bd91b314af5c54
Signed-off-by: JP Abgrall <jpa@google.com>
ping: prevent NULL pointer dereference on write to msg_name

A plain read() on a socket does set msg->msg_name to NULL. So check for
NULL pointer first.

[Backport of net-next cf970c002d270c36202bd5b9c2804d3097a52da0]

Bug: 12780426
Change-Id: Ida66892d359d2dfbb32803d8b0872bf8c0bb3865
Signed-off-by: Hannes Frederic Sowa <hannes@stressinduktion.org>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Lorenzo Colitti <lorenzo@google.com>