keys: Provide the original description to the key preparser

Provide the proposed description (add key) or the original description
(update/instantiate key) when preparsing a key so that the key type can
validate it against the data.

This is important for rxrpc server keys as we need to check that they have
the right amount of key material present - and it's better to do that when
the key is loaded rather than deep in trying to process a response packet.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Jarkko Sakkinen <jarkko.sakkinen@linux.intel.com>
cc: keyrings@vger.kernel.org