selftests: mptcp: make 2nd net namespace use tcp syn cookies unconditionally

check we can establish connections also when syn cookies are in use.

Check that
MPTcpExtMPCapableSYNRX and MPTcpExtMPCapableACKRX increase for each
MPTCP test.

Check TcpExtSyncookiesSent and TcpExtSyncookiesRecv increase in netns2.

Signed-off-by: Florian Westphal <fw@strlen.de>
Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>