tracing: Use __this_cpu_read() in trace_event_buffer_lock_reserver()

The value read by this_cpu_read() is used later and its use is expected to
stay on the same CPU as being read. But this_cpu_read() does not warn if
it is called without preemption disabled, where as __this_cpu_read() will
check if preemption is disabled on CONFIG_DEBUG_PREEMPT

Currently all callers have preemption disabled, but there may be new
callers in the future that may not.

Link: https://lkml.kernel.org/r/20211130024318.698165354@goodmis.orgSigned-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

kernel/trace/trace.c

@@ -2746,7 +2746,7 @@ trace_event_buffer_lock_reserve(struct trace_buffer **current_rb,
 
 	if (!tr->no_filter_buffering_ref &&
 	    (trace_file->flags & (EVENT_FILE_FL_SOFT_DISABLED | EVENT_FILE_FL_FILTERED)) &&
-	    (entry = this_cpu_read(trace_buffered_event))) {
+	    (entry = __this_cpu_read(trace_buffered_event))) {
 		/*
 		 * Filtering is on, so try to use the per cpu buffer first.
 		 * This buffer will simulate a ring_buffer_event,