iproute2: Two backports to allow clang building

Summary:
Squashed commit of the following to allow
clang building of iproute2

iproute2: allow to specify truncation bits on auth algo

Hi,

here is a patch against iproute2 to allow user to set a state with a specific
auth length.

Example:
$ ip xfrm state add src 10.16.0.72 dst 10.16.0.121 proto ah spi 0x10000000
auth-trunc "sha256" "azertyuiopqsdfghjklmwxcvbn123456" 96 mode tunnel
$ ip xfrm state
src 10.16.0.72 dst 10.16.0.121
         proto ah spi 0x10000000 reqid 0 mode tunnel
         replay-window 0
         auth-trunc hmac(sha256)
0x617a6572747975696f707173646667686a6b6c6d77786376626e313233343536 96
         sel src 0.0.0.0/0 dst 0.0.0.0/0

Regards,
Nicolas

>From 522ed7348cdf3b6f501af2a5a5d989de1696565a Mon Sep 17 00:00:00 2001
From: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Date: Thu, 23 Dec 2010 06:48:12 -0500
Subject: [PATCH] iproute2: allow to specify truncation bits on auth algo

Attribute XFRMA_ALG_AUTH_TRUNC can be used to specify
truncation bits, so we add a new algo type: auth-trunc.

Signed-off-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>

ipxfrm: use alloca to allocate stack space

Clang doesn't support the gcc extension for embeddeding flexible arrays
inside of structures.  Use the slightly more portable alloca().

Signed-off-by: Mike Frysinger <vapier@gentoo.org>

Test Plan: verified

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D645

Change-Id: If8622070f978d09d2cca1f3ff4d489b4a440c1ba