selftests: mptcp: join: helper to filter TCP

This is more readable and reduces duplicated commands.

This might also be useful to add v6 support and switch to nftables.
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -561,6 +561,15 @@ pm_nl_check_endpoint()
 	fi
 }
 
+filter_tcp_from()
+{
+	local ns="${1}"
+	local src="${2}"
+	local target="${3}"
+
+	ip netns exec "${ns}" iptables -A INPUT -s "${src}" -p tcp -j "${target}"
+}
+
 do_transfer()
 {
 	listener_ns="$1"
@@ -1519,7 +1528,7 @@ subflows_error_tests()
 		pm_nl_set_limits $ns2 0 2
 		pm_nl_add_endpoint $ns2 10.0.3.2 flags subflow
 		pm_nl_add_endpoint $ns2 10.0.2.2 flags subflow
-		ip netns exec $ns1 iptables -A INPUT -s 10.0.3.2 -p tcp -j REJECT
+		filter_tcp_from $ns1 10.0.3.2 REJECT
 		run_tests $ns1 $ns2 10.0.1.1 0 0 0 slow
 		chk_join_nr 1 1 1
 	fi
@@ -1530,7 +1539,7 @@ subflows_error_tests()
 		pm_nl_set_limits $ns2 0 2
 		pm_nl_add_endpoint $ns2 10.0.3.2 flags subflow
 		pm_nl_add_endpoint $ns2 10.0.2.2 flags subflow
-		ip netns exec $ns1 iptables -A INPUT -s 10.0.3.2 -p tcp -j DROP
+		filter_tcp_from $ns1 10.0.3.2 DROP
 		run_tests $ns1 $ns2 10.0.1.1 0 0 0 slow
 		chk_join_nr 1 1 1
 	fi
@@ -1542,7 +1551,7 @@ subflows_error_tests()
 		pm_nl_set_limits $ns1 0 1
 		pm_nl_set_limits $ns2 0 1
 		pm_nl_add_endpoint $ns2 10.0.3.2 flags subflow
-		ip netns exec $ns1 iptables -A INPUT -s 10.0.3.2 -p tcp -j REJECT
+		filter_tcp_from $ns1 10.0.3.2 REJECT
 		run_tests $ns1 $ns2 10.0.1.1 0 0 0 slow &
 
 		# mpj subflow will be in TW after the reset