xfrm: Define new XFRM netlink auth attribute with specified truncation bits

The new XFRMA_ALG_AUTH_TRUNC attribute taking a xfrm_algo_auth as
argument allows the installation of authentication algorithms with
a truncation length specified in userspace, i.e. SHA256 with 128 bit
instead of 96 bit truncation.

Signed-off-by: Martin Willi <martin@strongswan.org>
Signed-off-by: David S. Miller <davem@davemloft.net>