[IPSEC]: Use digest_null directly for auth

Previously digest_null had no setkey function which meant that
we used hmac(digest_null) for IPsec since IPsec always calls
setkey.  Now that digest_null has a setkey we no longer need to
do that.

In fact when only confidentiality is specified for ESP we already
use digest_null directly.  However, when the null algorithm is
explicitly specified by the user we still opt for hmac(digest_null).

This patch removes this discrepancy.  I have not added a new compat
name for it because by chance it wasn't actualy possible for the user
to specify the name hmac(digest_null) due to a key length check in
xfrm_user (which I found out when testing that compat name :)

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