rcu-tasks: Make RCU Tasks Trace stall warnings print full .b.need_qs field

Currently, the RCU Tasks Trace CPU stall warning simply indicates
whether or not the .b.need_qs field is zero.  This commit shows the
three permitted values and flags other values with either "!" or "?".
This is a debugging aid.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Eric Dumazet <edumazet@google.com>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Martin KaFai Lau <kafai@fb.com>
Cc: KP Singh <kpsingh@kernel.org>

kernel/rcu/tasks.h

@@ -1502,13 +1502,14 @@ static void show_stalled_task_trace(struct task_struct *t, bool *firstreport)
 			 ".I"[t->trc_ipi_to_cpu >= 0],
 			 ".i"[is_idle_tsk]);
 	else
-		pr_alert("P%d: %c%c%c nesting: %d%c cpu: %d%s\n",
+		pr_alert("P%d: %c%c%c nesting: %d%c%c cpu: %d%s\n",
 			 t->pid,
 			 ".I"[trc_rdr.ipi_to_cpu >= 0],
 			 ".i"[is_idle_tsk],
 			 ".N"[cpu >= 0 && tick_nohz_full_cpu(cpu)],
 			 trc_rdr.nesting,
-			 " N"[!!trc_rdr.needqs],
+			 " !CN"[trc_rdr.needqs & 0x3],
+			 " ?"[trc_rdr.needqs > 0x3],
 			 cpu, cpu_online(cpu) ? "" : "(offline)");
 	sched_show_task(t);
 }