softirq: Avoid bad tracing / lockdep interaction

Similar to commit:

  1a63dcd ("softirq: Reorder trace_softirqs_on to prevent lockdep splat")

__local_bh_enable_ip() can also call into tracing with inconsistent
state. Unlike that commit we don't need to bother about the tracepoint
because 'cnt-1' never matches preempt_count() (by construction).

Reported-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Heiko Carstens <hca@linux.ibm.com>
Link: https://lkml.kernel.org/r/20201218154519.GW3092@hirez.programming.kicks-ass.net

Author: Peter Zijlstra

kernel/softirq.c

@@ -185,7 +185,7 @@ void __local_bh_enable_ip(unsigned long ip, unsigned int cnt)
 	 * Keep preemption disabled until we are done with
 	 * softirq processing:
 	 */
-	preempt_count_sub(cnt - 1);
+	__preempt_count_sub(cnt - 1);
 
 	if (unlikely(!in_interrupt() && local_softirq_pending())) {
 		/*