rxrpc: Fix rxkad token xdr encoding

rxrpc: Fix rxkad token xdr encoding

The session key should be encoded with just the 8 data bytes and
no length; ENCODE_DATA precedes it with a 4 byte length, which
confuses some existing tools that try to parse this format.

Add an ENCODE_BYTES macro that does not include a length, and use
it for the key.  Also adjust the expected length.

Note that commit eb012c9d5ef93 ("rxrpc: Fix an assertion in
rxrpc_read()") had fixed a BUG by changing the length rather than
fixing the encoding.  The original length was correct.

Fixes: 35cef4351aa0 ("RxRPC: Parse security index 5 keys (Kerberos 5)")
Signed-off-by: Marc Dionne <marc.dionne@auristor.com>
Signed-off-by: David Howells <dhowells@redhat.com>