Actually machine__exit forgot to call machine__destroy_kernel_maps.

This fixes some memory leaks on map as below.

Without this fix.
  ----
  ./perf probe vfs_read
  Added new event:
    probe:vfs_read       (on vfs_read)

  You can now use it in all perf tools, such as:

          perf record -e probe:vfs_read -aR sleep 1

  REFCNT: BUG: Unreclaimed objects found.
  REFCNT: Total 4 objects are not reclaimed.
     To see all backtraces, rerun with -v option
  ----
With this fix.
  ----
  ./perf probe vfs_read
  Added new event:
    probe:vfs_read       (on vfs_read)

  You can now use it in all perf tools, such as:

          perf record -e probe:vfs_read -aR sleep 1

  REFCNT: BUG: Unreclaimed objects found.
  REFCNT: Total 2 objects are not reclaimed.
     To see all backtraces, rerun with -v option
  ----
Signed-off-by: Masami Hiramatsu <masami.hiramatsu.pt@hitachi.com>
Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20151118064024.30709.43577.stgit@localhost.localdomainSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>