perf probe: Fix bug with global variables handling

perf probe: Fix bug with global variables handling

There are missing curly braces which causes find_variable() return wrong
value when probing with global variables.

This problem can be reproduced as following:

  $ perf probe -v --add='generic_perform_write global_variable_for_test'
  ...
  Try to find probe point from debuginfo.
  Probe point found: generic_perform_write+0
  Searching 'global_variable_for_test' variable in context.
  An error occurred in debuginfo analysis (-2).
    Error: Failed to add events. Reason: No such file or directory (Code: -2)

After this patch:

  $ perf probe -v --add='generic_perform_write global_variable_for_test'
  ...
  Converting variable global_variable_for_test into trace event.
  global_variable_for_test type is int.
  Found 1 probe_trace_events.
  Opening /sys/kernel/debug/tracing/kprobe_events write=1
  Added new event:
  Writing event: p:probe/generic_perform_write _stext+1237464
  global_variable_for_test=@global_variable_for_test+0:s32
    probe:generic_perform_write (on generic_perform_write with
    global_variable_for_test)

  You can now use it in all perf tools, such as:

      perf record -e probe:generic_perform_write -aR sleep 1

Signed-off-by: He Kuang <hekuang@huawei.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.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/1429949338-18678-1-git-send-email-hekuang@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>