perf intel-pt: Fix segfault tracing transactions

perf intel-pt: Fix segfault tracing transactions

Tracing a workload that uses transactions gave a seg fault as follows:

  perf record -e intel_pt// workload
  perf report
  Program received signal SIGSEGV, Segmentation fault.
  0x000000000054b58c in intel_pt_reset_last_branch_rb (ptq=0x1a36110)
  	at util/intel-pt.c:929
  929 ptq->last_branch_rb->nr = 0;
  (gdb) p ptq->last_branch_rb
  $1 = (struct branch_stack *) 0x0
  (gdb) up
  1148 intel_pt_reset_last_branch_rb(ptq);
  (gdb) l
  1143 if (ret)
  1144 pr_err("Intel Processor Trace: failed to deliver transaction event
  1145 ret);
  1146
  1147 if (pt->synth_opts.callchain)
  1148 intel_pt_reset_last_branch_rb(ptq);
  1149
  1150 return ret;
  1151 }
  1152
  (gdb) p pt->synth_opts.callchain
  $2 = true
  (gdb)
  (gdb) bt
   #0 0x000000000054b58c in intel_pt_reset_last_branch_rb (ptq=0x1a36110)
   #1 0x000000000054c1e0 in intel_pt_synth_transaction_sample (ptq=0x1a36110)
   #2 0x000000000054c5b2 in intel_pt_sample (ptq=0x1a36110)

Caused by checking the 'callchain' flag when it should have been the
'last_branch' flag.  Fix that.

Reported-by: Andi Kleen <ak@linux.intel.com>
Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: stable@vger.kernel.org # v4.4+
Fixes: f14445ee72c5 ("perf intel-pt: Support generating branch stack")
Link: http://lkml.kernel.org/r/1460977068-11566-1-git-send-email-adrian.hunter@intel.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>