selftests: rtnetlink: hide complaint from terminated monitor

Set up the "ip xfrm monitor" subprogram so as to not see
a "Terminated" message when the subprogram is killed.

Fixes: 5e596ee1

 ("selftests: add xfrm state-policy-monitor to rtnetlink.sh")
Reported-by: Anders Roxell <anders.roxell@linaro.org>
Signed-off-by: Shannon Nelson <shannon.nelson@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>