psp: validate IPv4 header fields in psp_dev_rcv()

psp: validate IPv4 header fields in psp_dev_rcv()

psp_dev_rcv() is called from the NIC driver's RX completion path
before the frame reaches ip_rcv_core(), so the IP header has not
been validated in SW, yet. We expect that the device has done
all this validation, but let's also add the SW checks, to avoid
surprises.

Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20260428205352.1247325-4-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>