selftests: mptcp: pcap: drop most of the payload

Limit the size of each captured packet to 108B (IPv4 only) or 128B (a
mix of v4 and v6): this should drop most of the payload that is
generally not needed when debugging an issue.

8 bytes are left in this payload, to be able to inspect the beginning,
just in case.

Please also note that generally, this payload is usually mostly filled
with 0, except at the end. This reduces the .pcap sizes, and reduce IO
usage, which helps debugging issues.

Reviewed-by: Mat Martineau <martineau@kernel.org>
Signed-off-by: Matthieu Baerts (NGI0) <matttbe@kernel.org>
Link: https://patch.msgid.link/20260812-net-next-mptcp-misc-feat-7-3-v1-9-1905a818f6cb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

tools/testing/selftests/net/mptcp/mptcp_connect.sh

@@ -373,7 +373,7 @@ do_transfer()
 		fi
 
 		local capfile="${capprefix}-${connector_ns:0:3}-${listener_ns:0:3}-${cl_proto}-${srv_proto}-${connect_addr}-${port}"
-		local capopt="-i any -s 65535 -B 32768 ${capuser}"
+		local capopt="-i any -s 128 -B 32768 ${capuser}"
 
 		ip netns exec ${listener_ns} tcpdump ${capopt} \
 			-w "${capfile}-listener.pcap" >> "${capout}" 2>&1 &

tools/testing/selftests/net/mptcp/mptcp_join.sh

@@ -979,7 +979,7 @@ cond_start_capture()
 		capfile=$(printf "mp_join-%02u-%s.pcap" "$MPTCP_LIB_TEST_COUNTER" "$ns")
 
 		echo "Capturing traffic for test $MPTCP_LIB_TEST_COUNTER into $capfile"
-		ip netns exec "$ns" tcpdump -i any -s 65535 -B 32768 $capuser -w "$capfile" > "$capout" 2>&1 &
+		ip netns exec "$ns" tcpdump -i any -s 128 -B 32768 $capuser -w "$capfile" > "$capout" 2>&1 &
 		cappid=$!
 
 		sleep 1

tools/testing/selftests/net/mptcp/simult_flows.sh

@@ -149,7 +149,7 @@ do_transfer()
 		fi
 
 		local capfile="${capprefix}-${port}"
-		local capopt="-i any -s 65535 -B 32768 ${capuser}"
+		local capopt="-i any -s 108 -B 32768 ${capuser}"
 
 		ip netns exec ${ns3}  tcpdump ${capopt} -w "${capfile}-listener.pcap"  >> "${capout}" 2>&1 &
 		local cappid_listener=$!