selftests: mptcp: sockopt: use 'iptables-legacy' if available

IPTables commands using 'iptables-nft' fail on old kernels, at least
on v5.15 because it doesn't see the default IPTables chains:

  $ iptables -L
  iptables/1.8.2 Failed to initialize nft: Protocol not supported

As a first step before switching to NFTables, we can use iptables-legacy
if available.

Link: https://github.com/multipath-tcp/mptcp_net-next/issues/368
Fixes: dc65fe82fb ("selftests: mptcp: add packet mark test case")
Cc: stable@vger.kernel.org
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

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

@@ -12,6 +12,8 @@ ksft_skip=4
 timeout_poll=30
 timeout_test=$((timeout_poll * 2 + 1))
 mptcp_connect=""
+iptables="iptables"
+ip6tables="ip6tables"
 
 sec=$(date +%s)
 rndh=$(printf %x $sec)-$(mktemp -u XXXXXX)
@@ -25,7 +27,7 @@ add_mark_rules()
 	local m=$2
 
 	local t
-	for t in iptables ip6tables; do
+	for t in ${iptables} ${ip6tables}; do
 		# just to debug: check we have multiple subflows connection requests
 		ip netns exec $ns $t -A OUTPUT -p tcp --syn -m mark --mark $m -j ACCEPT
 
@@ -95,14 +97,14 @@ if [ $? -ne 0 ];then
 	exit $ksft_skip
 fi
 
-iptables -V > /dev/null 2>&1
-if [ $? -ne 0 ];then
+# Use the legacy version if available to support old kernel versions
+if iptables-legacy -V &> /dev/null; then
+	iptables="iptables-legacy"
+	ip6tables="ip6tables-legacy"
+elif ! iptables -V &> /dev/null; then
 	echo "SKIP: Could not run all tests without iptables tool"
 	exit $ksft_skip
-fi
-
-ip6tables -V > /dev/null 2>&1
-if [ $? -ne 0 ];then
+elif ! ip6tables -V &> /dev/null; then
 	echo "SKIP: Could not run all tests without ip6tables tool"
 	exit $ksft_skip
 fi
@@ -112,10 +114,10 @@ check_mark()
 	local ns=$1
 	local af=$2
 
-	local tables=iptables
+	local tables=${iptables}
 
 	if [ $af -eq 6 ];then
-		tables=ip6tables
+		tables=${ip6tables}
 	fi
 
 	local counters values