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: 5e596ee171ba ("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>

tools/testing/selftests/net/rtnetlink.sh

@@ -535,8 +535,7 @@ kci_test_ipsec()
 
 	# start the monitor in the background
 	tmpfile=`mktemp ipsectestXXX`
-	ip x m > $tmpfile &
-	mpid=$!
+	mpid=`(ip x m > $tmpfile & echo $!) 2>/dev/null`
 	sleep 0.2
 
 	ipsecid="proto esp src $srcip dst $dstip spi 0x07"