xfrm: Add SHA384 and SHA512 HMAC authentication algorithms to XFRM

These algorithms use a truncation of 192/256 bits, as specified
in RFC4868.

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>