selftests/net: packetdrill: add --mss option to three tests

selftests/net: packetdrill: add --mss option to three tests

Three tests are cooking GSO packets but do not provide
gso_size information to the kernel, triggering this message:

TCP: tun0: Driver has suspect GRO implementation, TCP performance may be compromised.

Add --mss option to avoid this warning.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Link: https://patch.msgid.link/20250710155641.3028726-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>