dev_refthread: Do not initialize *ref when reference was not acquired

Like the companion API devvn_refthread, leave *ref uninitialized when a
reference was not acquired.  Initializing to 1 provides a vaguely
correct-looking but bogus value for broken callers to (mistakenly) pass to
dev_relthread() when refthread fails.

Make it even more clear to consumers that dev_relthread is only valid when
dev_refthread succeeds.

Reviewed by:	kib, markj
Sponsored by:	Dell EMC Isilon
Differential Revision:	https://reviews.freebsd.org/D16885

sys/kern/kern_conf.c

@@ -202,7 +202,8 @@ dev_refthread(struct cdev *dev, int *ref)
 			csw = NULL;
 	}
 	dev_unlock();
-	*ref = 1;
+	if (csw != NULL)
+		*ref = 1;
 	return (csw);
 }