psp: track generations of device key

There is a (somewhat theoretical in absence of multi-host support)
possibility that another entity will rotate the key and we won't
know. This may lead to accepting packets with matching SPI but
which used different crypto keys than we expected.

The PSP Architecture specification mentions that an implementation
should track device key generation when device keys are managed by the
NIC. Some PSP implementations may opt to include this key generation
state in decryption metadata each time a device key is used to decrypt
a packet. If that is the case, that key generation counter can also be
used when policy checking a decrypted skb against a psp_assoc. This is
an optional feature that is not explicitly part of the PSP spec, but
can provide additional security in the case where an attacker may have
the ability to force key rotations faster than rekeying can occur.

Since we're tracking "key generations" more explicitly now,
maintain different lists for associations from different generations.
This way we can catch stale associations (the user space should
listen to rotation notifications and change the keys).

Drivers can "opt out" of generation tracking by setting
the generation value to 0.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250917000954.859376-11-daniel.zahka@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>

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