perf probe: Begin and end libdwfl report session correctly

Fix a trival bug about libdwfl usage of the report session, it should
explicitly begin and end a report session around dwfl_report_offline().

Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/20150930164128.3733.59876.stgit@localhost.localdomain


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