crypto/krb5: Implement the Kerberos5 rfc3961 encrypt and decrypt functions

crypto/krb5: Implement the Kerberos5 rfc3961 encrypt and decrypt functions

Add functions that encrypt and decrypt a message according to rfc3961 sec
5.3, using Ki to checksum the data to be secured and Ke to encrypt it
during the encryption phase, then decrypting with Ke and verifying the
checksum with Ki in the decryption phase.

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