perf top: Convert to hist_entry_iter

perf top: Convert to hist_entry_iter

Reuse hist_entry_iter__add() function to share the similar code with
perf report.  Note that it needs to be called with hists.lock so tweak
some internal functions not to deadlock or hold the lock too long.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Tested-by: Arun Sharma <asharma@fb.com>
Tested-by: Rodrigo Campos <rodrigo@sdfg.com.ar>
Link: http://lkml.kernel.org/r/1401335910-16832-20-git-send-email-namhyung@kernel.org
Signed-off-by: Jiri Olsa <jolsa@kernel.org>