lockdep: Fix nr_unused_locks accounting

Chris reported that commit 24d5a3bffef1 ("lockdep: Fix
usage_traceoverflow") breaks the nr_unused_locks validation code
triggered by /proc/lockdep_stats.

By fully splitting LOCK_USED and LOCK_USED_READ it becomes a bad
indicator for accounting nr_unused_locks; simplyfy by using any first
bit.

Fixes: 24d5a3bffef1 ("lockdep: Fix usage_traceoverflow")
Reported-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://lkml.kernel.org/r/20201027124834.GL2628@hirez.programming.kicks-ass.net

kernel/locking/lockdep.c

@@ -4396,26 +4396,20 @@ static int mark_lock(struct task_struct *curr, struct held_lock *this,
 	if (unlikely(hlock_class(this)->usage_mask & new_mask))
 		goto unlock;
 
+	if (!hlock_class(this)->usage_mask)
+		debug_atomic_dec(nr_unused_locks);
+
 	hlock_class(this)->usage_mask |= new_mask;
 
 	if (new_bit < LOCK_TRACE_STATES) {
 		if (!(hlock_class(this)->usage_traces[new_bit] = save_trace()))
 			return 0;
 	}
 
-	switch (new_bit) {
-	case 0 ... LOCK_USED-1:
+	if (new_bit < LOCK_USED) {
 		ret = mark_lock_irq(curr, this, new_bit);
 		if (!ret)
 			return 0;
-		break;
-
-	case LOCK_USED:
-		debug_atomic_dec(nr_unused_locks);
-		break;
-
-	default:
-		break;
 	}
 
 unlock: