lockdep: Fix redundant_hardirqs_on incremented with irqs enabled

When a path restore the flags while irqs are already enabled, we
update the per cpu var redundant_hardirqs_on in a racy fashion
and debug_atomic_inc() warns about this situation.

In this particular case, loosing a few hits in a stat is not a big
deal, so increment it without protection.

v2: Don't bother with disabling irq, we can miss one count in
    rare situations
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: David Miller <davem@davemloft.net>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>

kernel/lockdep.c

@@ -2298,7 +2298,12 @@ void trace_hardirqs_on_caller(unsigned long ip)
 		return;
 
 	if (unlikely(curr->hardirqs_enabled)) {
-		debug_atomic_inc(redundant_hardirqs_on);
+		/*
+		 * Neither irq nor preemption are disabled here
+		 * so this is racy by nature but loosing one hit
+		 * in a stat is not a big deal.
+		 */
+		this_cpu_inc(lockdep_stats.redundant_hardirqs_on);
 		return;
 	}
 	/* we'll do an OFF -> ON transition: */