Return a reasonable number for top or ps to display for M:N thread,

since there is no direct association between M:N thread and kse,
sometimes, a thread does not have a kse, in that case, return a pctcpu
from its last kse, it is not perfect, but gives a good number to be
displayed.

sys/kern/sched_4bsd.c

@@ -723,6 +723,8 @@ sched_pctcpu(struct thread *td)
 	struct kse *ke;
 
 	ke = td->td_kse;
+	if (ke == NULL)
+		ke = td->td_last_kse;
 	if (ke)
 		return (ke->ke_pctcpu);