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

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: 0c4cd3f86a40 ("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.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>