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>
Git-commit: 969ff3bbb38b6622800a1a4bd38404e3701193de
Git-Repo: https://android.googlesource.com/kernel/common.git


[imaund@codeaurora.org: Resolve merge conflicts]
Signed-off-by: Ian Maund <imaund@codeaurora.org>