perf tests: Set evlist of test__backward_ring_buffer() to !overwrite

perf tests: Set evlist of test__backward_ring_buffer() to !overwrite

Setting overwrite in perf_evlist__mmap() is meaningless because the
event in this evlist is already have 'overwrite' postfix and goes to
backward ring buffer automatically. Pass 'false' to perf_evlist__mmap()
to make it similar to others.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kan Liang <kan.liang@intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20171113013809.212417-3-wangnan0@huawei.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>