Automatically detect deadlock condition in destroy_dev(), that is, if

destroy_dev() is called from csw method, and no d_purge driver method is
provided. Transform the direct call to destroy_dev() into destroy_dev_sched().

Reviewed by:	njl (programming interface)
Debugging help and testing by:	Peter Holm
Approved by:	re (kensmith)

sys/kern/kern_conf.c

@@ -779,10 +779,16 @@ destroy_devl(struct cdev *dev)
 void
 destroy_dev(struct cdev *dev)
 {
+	struct cdevsw *csw;
 
 	dev_lock();
-	destroy_devl(dev);
-	dev_unlock_and_free();
+	csw = dev->si_devsw;
+	if ((csw != NULL && csw->d_purge != NULL) ||
+	    dev->si_threadcount == 0) {
+		destroy_devl(dev);
+		dev_unlock_and_free();
+	} else
+		destroy_dev_sched(dev);
 }
 
 const char *