tracing: Use in_serving_softirq() to deduct softirq status.

PREEMPT_RT does not report "serving softirq" because the tracing core
looks at the preemption counter while PREEMPT_RT does not update it
while processing softirqs in order to remain preemptible. The
information is stored somewhere else.
The in_serving_softirq() macro and the SOFTIRQ_OFFSET define are still
working but not on the preempt-counter.

Use in_serving_softirq() macro which works on PREEMPT_RT. On !PREEMPT_RT
the compiler (gcc-10 / clang-11) is smart enough to optimize the
in_serving_softirq() related read of the preemption counter away.
The only difference I noticed by using in_serving_softirq() on
!PREEMPT_RT is that gcc-10 implemented tracing_gen_ctx_flags() as
reading FLAG, jmp _tracing_gen_ctx_flags(). Without in_serving_softirq()
it inlined _tracing_gen_ctx_flags() into tracing_gen_ctx_flags().

Link: https://lkml.kernel.org/r/20210125194511.3924915-4-bigeasy@linutronix.deSigned-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -2590,8 +2590,7 @@ unsigned int tracing_gen_ctx_irq_test(unsigned int irqs_status)
 		trace_flags |= TRACE_FLAG_NMI;
 	if (pc & HARDIRQ_MASK)
 		trace_flags |= TRACE_FLAG_HARDIRQ;
-
-	if (pc & SOFTIRQ_OFFSET)
+	if (in_serving_softirq())
 		trace_flags |= TRACE_FLAG_SOFTIRQ;
 
 	if (tif_need_resched())