perf probe: Remove length limitation for showing available variables

Use struct strbuf instead of bare char[] to remove the length limitation
of variables in variable_list, so they will not disappear due to
overlength, and make preparation for adding more description for
variables.

Signed-off-by: He Kuang <hekuang@huawei.com>
Acked-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1431336304-16863-1-git-send-email-hekuang@huawei.com


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