perf callchain: Reset cursor arg instead of callchain_cursor

perf callchain: Reset cursor arg instead of callchain_cursor

We already pass cursor into thread__resolve_callchain function, so
there's no point in resetting the global instance.

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/n/tip-puk015qvuppao9m1xtdy9v7j@git.kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>