selftests: mptcp: fix proto type in link_failure tests

In listener_ns, we should pass srv_proto argument to mptcp_connect command,
not cl_proto.

Fixes: 7d1e6f16 ("selftests: mptcp: add testcase for active-back")
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Matthieu Baerts <matthieu.baerts@tessares.net>
Signed-off-by: David S. Miller <davem@davemloft.net>