xfrm: Fix truncation length of authentication algorithms installed via PF_KEY

Commit 4447bb33 ("xfrm: Store aalg in
xfrm_state with a user specified truncation length") breaks
installation of authentication algorithms via PF_KEY, as the state
specific truncation length is not installed with the algorithms
default truncation length.  This patch initializes state properly to
the default if installed via PF_KEY.

Signed-off-by: Martin Willi <martin@strongswan.org>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>