selftests: mptcp: avoid Terminated messages in userspace_pm

There're some 'Terminated' messages in the output of userspace pm tests
script after killing './pm_nl_ctl events' processes:

Created network namespaces ns1, ns2         			[OK]
./userspace_pm.sh: line 166: 13735 Terminated              ip netns exec "$ns2" ./pm_nl_ctl events >> "$client_evts" 2>&1
./userspace_pm.sh: line 172: 13737 Terminated              ip netns exec "$ns1" ./pm_nl_ctl events >> "$server_evts" 2>&1
Established IPv4 MPTCP Connection ns2 => ns1    		[OK]
./userspace_pm.sh: line 166: 13753 Terminated              ip netns exec "$ns2" ./pm_nl_ctl events >> "$client_evts" 2>&1
./userspace_pm.sh: line 172: 13755 Terminated              ip netns exec "$ns1" ./pm_nl_ctl events >> "$server_evts" 2>&1
Established IPv6 MPTCP Connection ns2 => ns1    		[OK]
ADD_ADDR 10.0.2.2 (ns2) => ns1, invalid token    		[OK]

This patch adds a helper kill_wait(), in it using 'wait $pid 2>/dev/null'
commands after 'kill $pid' to avoid printing out these Terminated messages.
Use this helper instead of using 'kill $pid'.

Reviewed-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <mathew.j.martineau@linux.intel.com>
Signed-off-by: David S. Miller <davem@davemloft.net>