perf test: Update mmap read functions for backward-ring-buffer test

perf test: Update mmap read functions for backward-ring-buffer test

Use the new perf_mmap__read_* interfaces for overwrite ringbuffer test.

Commiter notes:

Testing:

  [root@seventh ~]# perf test -v backward
  48: Read backward ring buffer                             :
  --- start ---
  test child forked, pid 8309
  Using CPUID GenuineIntel-6-9E
  mmap size 1052672B
  mmap size 8192B
  Finished reading overwrite ring buffer: rewind
  test child finished with 0
  ---- end ----
  Read backward ring buffer: Ok
  [root@seventh ~]#

Signed-off-by: Kan Liang <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Jin Yao <[email protected]>
Cc: Namhyung Kim <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Wang Nan <[email protected]>
Link: http://lkml.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>