perf record: Fix duplicated sideband events with Intel PT system wide tracing

Commit 0a892c1c ("perf record: Add dummy event during system wide
synthesis") reveals an issue with Intel PT system wide tracing.
Specifically that Intel PT already adds a dummy tracking event, and it
is not the first event.  Adding another dummy tracking event causes
duplicated sideband events.  Fix by checking for an existing dummy
tracking event first.

Example showing duplicated switch events:

 Before:

   # perf record -a -e intel_pt//u uname
   Linux
   [ perf record: Woken up 1 times to write data ]
   [ perf record: Captured and wrote 0.895 MB perf.data ]
   # perf script --no-itrace --show-switch-events | head
            swapper     0 [007]  6390.516222: PERF_RECORD_SWITCH_CPU_WIDE OUT preempt  next pid/tid:    11/11
            swapper     0 [007]  6390.516222: PERF_RECORD_SWITCH_CPU_WIDE OUT preempt  next pid/tid:    11/11
          rcu_sched    11 [007]  6390.516223: PERF_RECORD_SWITCH_CPU_WIDE IN           prev pid/tid:     0/0
          rcu_sched    11 [007]  6390.516224: PERF_RECORD_SWITCH_CPU_WIDE IN           prev pid/tid:     0/0
          rcu_sched    11 [007]  6390.516227: PERF_RECORD_SWITCH_CPU_WIDE OUT          next pid/tid:     0/0
          rcu_sched    11 [007]  6390.516227: PERF_RECORD_SWITCH_CPU_WIDE OUT          next pid/tid:     0/0
            swapper     0 [007]  6390.516228: PERF_RECORD_SWITCH_CPU_WIDE IN           prev pid/tid:    11/11
            swapper     0 [007]  6390.516228: PERF_RECORD_SWITCH_CPU_WIDE IN           prev pid/tid:    11/11
            swapper     0 [002]  6390.516415: PERF_RECORD_SWITCH_CPU_WIDE OUT preempt  next pid/tid:  5556/5559
            swapper     0 [002]  6390.516416: PERF_RECORD_SWITCH_CPU_WIDE OUT preempt  next pid/tid:  5556/5559

 After:

   # perf record -a -e intel_pt//u uname
   Linux
   [ perf record: Woken up 1 times to write data ]
   [ perf record: Captured and wrote 0.868 MB perf.data ]
   #  perf script --no-itrace --show-switch-events | head
            swapper     0 [005]  6450.567013: PERF_RECORD_SWITCH_CPU_WIDE OUT preempt  next pid/tid:  7179/7181
               perf  7181 [005]  6450.567014: PERF_RECORD_SWITCH_CPU_WIDE IN           prev pid/tid:     0/0
               perf  7181 [005]  6450.567028: PERF_RECORD_SWITCH_CPU_WIDE OUT          next pid/tid:     0/0
            swapper     0 [005]  6450.567029: PERF_RECORD_SWITCH_CPU_WIDE IN           prev pid/tid:  7179/7181
            swapper     0 [005]  6450.571699: PERF_RECORD_SWITCH_CPU_WIDE OUT preempt  next pid/tid:    11/11
          rcu_sched    11 [005]  6450.571700: PERF_RECORD_SWITCH_CPU_WIDE IN           prev pid/tid:     0/0
          rcu_sched    11 [005]  6450.571702: PERF_RECORD_SWITCH_CPU_WIDE OUT          next pid/tid:     0/0
            swapper     0 [005]  6450.571703: PERF_RECORD_SWITCH_CPU_WIDE IN           prev pid/tid:    11/11
            swapper     0 [005]  6450.579703: PERF_RECORD_SWITCH_CPU_WIDE OUT preempt  next pid/tid:    11/11
          rcu_sched    11 [005]  6450.579704: PERF_RECORD_SWITCH_CPU_WIDE IN           prev pid/tid:     0/0

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Link: http://lore.kernel.org/lkml/20200629091955.17090-3-adrian.hunter@intel.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>