tracing: do not disable interrupts for trace_clock_local

Disabling interrupts in trace_clock_local takes quite a performance
hit to the recording of traces. Using perf top we see:

------------------------------------------------------------------------------
   PerfTop:     244 irqs/sec  kernel:100.0% [1000Hz cpu-clock-msecs],  (all, 4 CPUs)
------------------------------------------------------------------------------

             samples    pcnt   kernel function
             _______   _____   _______________

             2842.00 - 40.4% : trace_clock_local
             1043.00 - 14.8% : rb_reserve_next_event
              784.00 - 11.1% : ring_buffer_lock_reserve
              600.00 -  8.5% : __rb_reserve_next
              579.00 -  8.2% : rb_end_commit
              440.00 -  6.3% : ring_buffer_unlock_commit
              290.00 -  4.1% : ring_buffer_producer_thread 	[ring_buffer_benchmark]
              155.00 -  2.2% : debug_smp_processor_id
              117.00 -  1.7% : trace_recursive_unlock
              103.00 -  1.5% : ring_buffer_event_data
               28.00 -  0.4% : do_gettimeofday
               22.00 -  0.3% : _spin_unlock_irq
               14.00 -  0.2% : native_read_tsc
               11.00 -  0.2% : getnstimeofday

Where trace_clock_local is 40% of the tracing, and the time for recording
a trace according to ring_buffer_benchmark is 210ns. After converting
the interrupts to preemption disabling we have from perf top:

------------------------------------------------------------------------------
   PerfTop:    1084 irqs/sec  kernel:99.9% [1000Hz cpu-clock-msecs],  (all, 4 CPUs)
------------------------------------------------------------------------------

             samples    pcnt   kernel function
             _______   _____   _______________

             1277.00 - 16.8% : native_read_tsc
             1148.00 - 15.1% : rb_reserve_next_event
              896.00 - 11.8% : ring_buffer_lock_reserve
              688.00 -  9.1% : __rb_reserve_next
              664.00 -  8.8% : rb_end_commit
              563.00 -  7.4% : ring_buffer_unlock_commit
              508.00 -  6.7% : _spin_unlock_irq
              365.00 -  4.8% : debug_smp_processor_id
              321.00 -  4.2% : trace_clock_local
              303.00 -  4.0% : ring_buffer_producer_thread 	[ring_buffer_benchmark]
              273.00 -  3.6% : native_sched_clock
              122.00 -  1.6% : trace_recursive_unlock
              113.00 -  1.5% : sched_clock
              101.00 -  1.3% : ring_buffer_event_data
               53.00 -  0.7% : tick_nohz_stop_sched_tick

Where trace_clock_local drops from 40% to only taking 4% of the total time.
The trace time also goes from 210ns down to 179ns (31ns).

I talked with Peter Zijlstra about the impact that sched_clock may have
without having interrupts disabled, and he told me that if a timer interrupt
comes in, sched_clock may report a wrong time.

Balancing a seldom incorrect timestamp with a 15% performance boost, I'll
take the performance boost.

Acked-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Steven Rostedt <rostedt@goodmis.org>