selftests: drv-net: psp: add test for auto-adjusting TCP MSS

Test TCP MSS getting auto-adjusted. PSP adds an encapsulation overhead
of 40B per packet, when used in transport mode without any
virtualization cookie or other optional PSP header fields. The kernel
should adjust the MSS for a connection after PSP tx state is reached.

Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Daniel Zahka <daniel.zahka@gmail.com>
Link: https://patch.msgid.link/20250927225420.1443468-8-kuba@kernel.org


Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>