crypto/krb5: Add an API to query the layout of the crypto section

crypto/krb5: Add an API to query the layout of the crypto section

Provide some functions to allow the called to find out about the layout of
the crypto section:

 (1) Calculate, for a given size of data, how big a buffer will be
     required to hold it and where the data will be within it.

 (2) Calculate, for an amount of buffer, what's the maximum size of data
     that will fit therein, and where it will start.

 (3) Determine where the data will be in a received message.

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