psp: provide decapsulation and receive helper for drivers

psp: provide decapsulation and receive helper for drivers

Create psp_dev_rcv(), which drivers can call to psp decapsulate and attach
a psp_skb_ext to an skb.

psp_dev_rcv() only supports what the PSP architecture specification
refers to as "transport mode" packets, where the L3 header is either
IPv6 or IPv4.

Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Raed Salem <raeds@nvidia.com>
Signed-off-by: Rahul Rameshbabu <rrameshbabu@nvidia.com>
Signed-off-by: Cosmin Ratiu <cratiu@nvidia.com>
Co-developed-by: Daniel Zahka <daniel.zahka@gmail.com>
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Link: https://patch.msgid.link/20250917000954.859376-18-daniel.zahka@gmail.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>