crypto/krb5: Provide RFC3961 setkey packaging functions

crypto/krb5: Provide RFC3961 setkey packaging functions

Provide functions to derive keys according to RFC3961 (or load the derived
keys for the selftester where only derived keys are available) and to
package them up appropriately for passing to a krb5enc AEAD setkey or a
hash setkey function.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: "David S. Miller" <davem@davemloft.net>
cc: Chuck Lever <chuck.lever@oracle.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
cc: linux-nfs@vger.kernel.org
cc: linux-crypto@vger.kernel.org
cc: netdev@vger.kernel.org