xfrm: Store aalg in xfrm_state with a user specified truncation length

Adding a xfrm_state requires an authentication algorithm specified
either as xfrm_algo or as xfrm_algo_auth with a specific truncation
length. For compatibility, both attributes are dumped to userspace,
and we also accept both attributes, but prefer the new syntax.

If no truncation length is specified, or the authentication algorithm
is specified using xfrm_algo, the truncation length from the algorithm
description in the kernel is used.

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