epoch: fix idle detection bug in synchronize.

An idle grace period requires all threads to be idle. This optimization
introduced a regression with idle detection if subset of threads are
both active and idle. Unfortunately, none of our test machines detected
the problem.

This issue was reported by Julie Zhao <julie.zhao@sparkpos....>

src/ck_epoch.c

@@ -309,11 +309,12 @@ ck_epoch_scan(struct ck_epoch *global,
 {
 	ck_stack_entry_t *cursor;
 
-	*af = false;
 	if (cr == NULL) {
 		cursor = CK_STACK_FIRST(&global->records);
+		*af = false;
 	} else {
 		cursor = &cr->record_next;
+		*af = true;
 	}
 
 	while (cursor != NULL) {