perf report: Fix branch stack mode cannot be set

perf report: Fix branch stack mode cannot be set

When perf.data file is obtained using 'perf record -b', perf report
should use branch stack mode to generate output. But this function is
broken by improper comparison between boolean and constant -1.

before this patch:

  $ perf report -b -i perf.data
  Samples: 16  of event 'cycles', Event count (approx.): 3171896
  Overhead  Command  Shared Object      Symbol
    13.59%  ls       [kernel.kallsyms]  [k] prio_tree_remove
    13.16%  ls       [kernel.kallsyms]  [k] change_pte_range
    12.09%  ls       [kernel.kallsyms]  [k] page_fault
    12.02%  ls       [kernel.kallsyms]  [k] zap_pte_range
  ...

after this patch:

  $ perf report -b -i perf.data
  Samples: 256  of event 'cycles', Event count (approx.): 256
  Overhead  Command  Source Shared Object  Source Symbol                               Target Shared Object  Target Symbol
     9.38%  ls       [unknown]             [k] 0000000000000000                        [unknown]             [k] 0000000000000000
     6.25%  ls       libc-2.19.so          [.] _dl_addr                                libc-2.19.so          [.] _dl_addr
     6.25%  ls       [kernel.kallsyms]     [k] zap_pte_range                           [kernel.kallsyms]     [k] zap_pte_range
     6.25%  ls       [kernel.kallsyms]     [k] change_pte_range                        [kernel.kallsyms]     [k] change_pte_range
     0.39%  ls       [kernel.kallsyms]     [k] prio_tree_remove                        [kernel.kallsyms]     [k] prio_tree_remove
  ...

Signed-off-by: He Kuang <hekuang@huawei.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1423967617-28879-1-git-send-email-hekuang@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>