selftests: mptcp: join: only check for ip6tables if needed

If 'iptables-legacy' is available, 'ip6tables-legacy' command will be
used instead of 'ip6tables'. So no need to look if 'ip6tables' is
available in this case.

Cc: stable@vger.kernel.org
Fixes: 0c4cd3f8 ("selftests: mptcp: join: use 'iptables-legacy' if available")
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20230725-send-net-20230725-v1-1-6f60fe7137a9@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

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

@@ -162,9 +162,7 @@ check_tools()
 	elif ! iptables -V &> /dev/null; then
 		echo "SKIP: Could not run all tests without iptables tool"
 		exit $ksft_skip
-	fi
-
-	if ! ip6tables -V &> /dev/null; then
+	elif ! ip6tables -V &> /dev/null; then
 		echo "SKIP: Could not run all tests without ip6tables tool"
 		exit $ksft_skip
 	fi