perf hists: Add helper functions for hierarchy mode

The rb_hierarchy_{next,prev,last} functions are to traverse all hist
entries in a hierarchy.  They will be used by various function which
supports hierarchy output.

As the rb_hierarchy_next() is used to traverse the whole hierarchy, it
sometime needs to visit entries regardless of current folding state.  So
add enum hierarchy_move_dir and pass it to __rb_hierarchy_next() for
those cases.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Acked-by: Pekka Enberg <penberg@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Stephane Eranian <eranian@google.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1456326830-30456-5-git-send-email-namhyung@kernel.org


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