afs: Use rxgk RESPONSE to pass token for callback channel

Implement in kafs the hook for adding appdata into a RESPONSE packet
generated in response to an RxGK CHALLENGE packet, and include the key for
securing the callback channel so that notifications from the fileserver get
encrypted.

This will be necessary when more complex notifications are used that convey
changed data around.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20250411095303.2316168-13-dhowells@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>