time/sched_clock: Mark sched_clock_read_begin/retry() as notrace

Since sched_clock_read_begin() and sched_clock_read_retry() are called
by notrace function sched_clock(), they shouldn't be traceable either,
or else ftrace_graph_caller will run into a dead loop on the path
as below (arm for instance):

  ftrace_graph_caller()
    prepare_ftrace_return()
      function_graph_enter()
        ftrace_push_return_trace()
          trace_clock_local()
            sched_clock()
              sched_clock_read_begin/retry()

Fixes: 1b86abc1 ("sched_clock: Expose struct clock_read_data")
Signed-off-by: Quanyang Wang <quanyang.wang@windriver.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/20200929082027.16787-1-quanyang.wang@windriver.com