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

commit 0c4cd3f86a40028845ad6f8af5b37165666404cd upstream.

IPTables commands using 'iptables-nft' fail on old kernels, at least
5.15 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: 8d014eaa92 ("selftests: mptcp: add ADD_ADDR timeout test case")
Cc: stable@vger.kernel.org
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -12,6 +12,8 @@ cinfail=""
 cinsent=""
 cout=""
 ksft_skip=4
+iptables="iptables"
+ip6tables="ip6tables"
 timeout_poll=30
 timeout_test=$((timeout_poll * 2 + 1))
 mptcp_connect=""
@@ -126,9 +128,9 @@ reset_with_add_addr_timeout()
 	local ip="${1:-4}"
 	local tables
 
-	tables="iptables"
+	tables="${iptables}"
 	if [ $ip -eq 6 ]; then
-		tables="ip6tables"
+		tables="${ip6tables}"
 	fi
 
 	reset
@@ -171,8 +173,10 @@ if [ $? -ne 0 ];then
 	exit $ksft_skip
 fi
 
-iptables -V > /dev/null 2>&1
-if [ $? -ne 0 ];then
+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