xfrm: Add support for per cpu xfrm state handling.

xfrm: Add support for per cpu xfrm state handling.

Currently all flows for a certain SA must be processed by the same
cpu to avoid packet reordering and lock contention of the xfrm
state lock.

To get rid of this limitation, the IETF standardized per cpu SAs
in RFC 9611. This patch implements the xfrm part of it.

We add the cpu as a lookup key for xfrm states and a config option
to generate acquire messages for each cpu.

With that, we can have on each cpu a SA with identical traffic selector
so that flows can be processed in parallel on all cpus.

Signed-off-by: Steffen Klassert <[email protected]>
Tested-by: Antony Antony <[email protected]>
Tested-by: Tobias Brunner <[email protected]>