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 <[e-mail]>
Reviewed-by: Willem de Bruijn <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

tools/testing/selftests/net/packetdrill/tcp_blocking_blocking-read.pkt

···
 // SPDX-License-Identifier: GPL-2.0
 // Test for blocking read.
+
 --tolerance_usecs=10000
+--mss=1000
 
 `./defaults.sh`
 

tools/testing/selftests/net/packetdrill/tcp_inq_client.pkt

···
 // SPDX-License-Identifier: GPL-2.0
 // Test TCP_INQ and TCP_CM_INQ on the client side.
+
+--mss=1000
+
 `./defaults.sh
 `
 

tools/testing/selftests/net/packetdrill/tcp_inq_server.pkt

···
 // SPDX-License-Identifier: GPL-2.0
 // Test TCP_INQ and TCP_CM_INQ on the server side.
+
+--mss=1000
+
 `./defaults.sh
 `