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

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>

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

@@ -25,6 +25,8 @@ capout=""
 ns1=""
 ns2=""
 ksft_skip=4
+iptables="iptables"
+ip6tables="ip6tables"
 timeout_poll=30
 timeout_test=$((timeout_poll * 2 + 1))
 capture=0
@@ -146,7 +148,11 @@ check_tools()
 		exit $ksft_skip
 	fi
 
-	if ! iptables -V &> /dev/null; 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
@@ -247,9 +253,9 @@ reset_with_add_addr_timeout()
 
 	reset "${1}" || return 1
 
-	tables="iptables"
+	tables="${iptables}"
 	if [ $ip -eq 6 ]; then
-		tables="ip6tables"
+		tables="${ip6tables}"
 	fi
 
 	ip netns exec $ns1 sysctl -q net.mptcp.add_addr_timeout=1
@@ -314,9 +320,9 @@ reset_with_fail()
 	local ip="${3:-4}"
 	local tables
 
-	tables="iptables"
+	tables="${iptables}"
 	if [ $ip -eq 6 ]; then
-		tables="ip6tables"
+		tables="${ip6tables}"
 	fi
 
 	ip netns exec $ns2 $tables \
@@ -704,7 +710,7 @@ filter_tcp_from()
 	local src="${2}"
 	local target="${3}"
 
-	ip netns exec "${ns}" iptables -A INPUT -s "${src}" -p tcp -j "${target}"
+	ip netns exec "${ns}" ${iptables} -A INPUT -s "${src}" -p tcp -j "${target}"
 }
 
 do_transfer()