This patch introduces the BEET mode (Bound End-to-End Tunnel) with as
specified by the ietf draft at the following link:

http://www.ietf.org/internet-drafts/draft-nikander-esp-beet-mode-06.txt

The patch provides only single family support (i.e. inner family =
outer family).
Signed-off-by: NDiego Beltrami <diego.beltrami@gmail.com>
Signed-off-by: NMiika Komu     <miika@iki.fi>
Signed-off-by: NHerbert Xu     <herbert@gondor.apana.org.au>
Signed-off-by: NAbhinav Pathak <abhinav.pathak@hiit.fi>
Signed-off-by: NJeff Ahrenholz <ahrenholz@gmail.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>