selftests: mptcp: display simult in extra_msg

Just like displaying "invert" after "Info: ", "simult" should be
displayed too when rm_subflow_nr doesn't match the expect value in
chk_rm_nr():

      syn                                 [ ok ]
      synack                              [ ok ]
      ack                                 [ ok ]
      add                                 [ ok ]
      echo                                [ ok ]
      rm                                  [ ok ]
      rmsf                                [ ok ] 3 in [2:4]
      Info: invert simult

      syn                                 [ ok ]
      synack                              [ ok ]
      ack                                 [ ok ]
      add                                 [ ok ]
      echo                                [ ok ]
      rm                                  [ ok ]
      rmsf                                [ ok ]
      Info: invert

Reviewed-by: Matthieu Baerts <matttbe@kernel.org>
Signed-off-by: Geliang Tang <geliang.tang@suse.com>
Signed-off-by: Mat Martineau <martineau@kernel.org>
Link: https://lore.kernel.org/r/20231025-send-net-next-20231025-v1-10-db8f25f798eb@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: geliangtang

tools/testing/selftests/net/mptcp/mptcp_join.sh

@@ -1770,7 +1770,10 @@ chk_rm_nr()
 		# in case of simult flush, the subflow removal count on each side is
 		# unreliable
 		count=$((count + cnt))
-		[ "$count" != "$rm_subflow_nr" ] && suffix="$count in [$rm_subflow_nr:$((rm_subflow_nr*2))]"
+		if [ "$count" != "$rm_subflow_nr" ]; then
+			suffix="$count in [$rm_subflow_nr:$((rm_subflow_nr*2))]"
+			extra_msg="$extra_msg simult"
+		fi
 		if [ $count -ge "$rm_subflow_nr" ] && \
 		   [ "$count" -le "$((rm_subflow_nr *2 ))" ]; then
 			print_ok "$suffix"