net: add mmsghdr struct check for L2TPV3

The mmsghdr struct is only introduced in Linux 2.6.32; add a
configure check for it and disable L2TPV3 on hosts which are
too old to provide it, rather than simply failing to compile.

Reported-by: chenliang <chenliang88@huawei.com>
Signed-off-by: Gonglei <arei.gonglei@huawei.com>
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Message-id: 1404219488-11196-1-git-send-email-arei.gonglei@huawei.com
[PMM: cleaned up commit message and corrected kernel version number]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
