netdevsim: psp: update rx stats on the peer netdevsim

netdevsim: psp: update rx stats on the peer netdevsim

nsim_do_psp() handles both tx and rx psp processing in the sending
device's nsim_start_xmit() path. The existing code has a logical bug,
where we erroneously increment rx_bytes and rx_packets on the sending
devices stats, instead of the peer device.

Additionally, compute psp_len after psp_dev_encapsulate() and before
psp_dev_rcv(), which modifies the header region of the skb. The
existing calculation was actually correct, because psp_dev_rcv()
leaves skb_inner_transport_header pointing at the tcp header, but this
is fragile and confusing as there is no actual inner transport header
after psp_dev_rcv has removed udp encapsulation.

Fixes: 178f0763c5f3 ("netdevsim: implement psp device stats")
Cc: <stable+noautosel@kernel.org> # netdevsim is a test harness, it's never loaded on production systems
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Link: https://patch.msgid.link/20260529-fix-psp-stats-v2-1-3a194eacf18e@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>