rcu-tasks: Simplify trc_inspect_reader() QS logic

Currently, trc_inspect_reader() does one check for nesting less than
or equal to zero, then sorts out the distinctions within this single
"if" statement.  This commit simplifies the logic by providing one
"if" statement for quiescent states (nesting of zero) and another "if"
statement for transitioning from one nesting level to another or the
outermost rcu_read_unlock_trace() (negative nesting).

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

@@ -1343,15 +1343,16 @@ static int trc_inspect_reader(struct task_struct *t, void *bhp_in)
 	// If not exiting a read-side critical section, mark as checked
 	// so that the grace-period kthread will remove it from the
 	// holdout list.
-	if (nesting <= 0) {
-		if (!nesting)
-			rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
-		return nesting ? -EINVAL : 0;  // If in QS, done, otherwise try again later.
+	if (!nesting) {
+		rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS_CHECKED);
+		return 0;  // In QS, so done.
 	}
+	if (nesting < 0)
+		return -EINVAL; //  QS transitioning, try again later.
 
 	// The task is in a read-side critical section, so set up its
-	// state so that it will awaken the grace-period kthread upon exit
-	// from that critical section.
+	// state so that it will update state upon exit from that critical
+	// section.
 	if (!rcu_trc_cmpxchg_need_qs(t, 0, TRC_NEED_QS | TRC_NEED_QS_CHECKED))
 		trc_add_holdout(t, bhp);
 	return 0;