rxrpc: rxgk: Implement connection rekeying

Implement rekeying of connections with the RxGK security class.  This
involves regenerating the keys with a different key number as part of the
input data after a certain amount of time or a certain amount of bytes
encrypted.  Rekeying may be triggered by either end.

The LSW of the key number is inserted into the security-specific field in
the RX header, and we try and expand it to 32-bits to make it last longer.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Marc Dionne <marc.dionne@auristor.com>
cc: Herbert Xu <herbert@gondor.apana.org.au>
cc: Chuck Lever <chuck.lever@oracle.com>
cc: Simon Horman <horms@kernel.org>
cc: linux-afs@lists.infradead.org
Link: https://patch.msgid.link/20250411095303.2316168-10-dhowells@redhat.com


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