[PATCH] Use WARN_ON in local_bh_enable()

This BUG_ON is triggering via ppp's line discipline flushing, due to
brokenness in tty_io.c.

We need to fix tty.  Meanwhile, let's not gratuitously nuke people's boxes.

kernel/softirq.c

@@ -102,7 +102,7 @@ asmlinkage void do_softirq(void)
 void local_bh_enable(void)
 {
 	__local_bh_enable();
-	BUG_ON(irqs_disabled());
+	WARN_ON(irqs_disabled());
 	if (unlikely(!in_interrupt() &&
 		     local_softirq_pending()))
 		invoke_softirq();