At the cost of an extra pointer, we can avoid the O(logN) cost of
finding the first element in the tree (smallest node), which is
something heavily required for histograms. Specifically, the following
are converted to rb_root_cached, and users accordingly:

hist::entries_in_array
hist::entries_in
hist::entries
hist::entries_collapsed
hist_entry::hroot_in
hist_entry::hroot_out
Signed-off-by: NDavidlohr Bueso <dbueso@suse.de>
Tested-by: NArnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/20181206191819.30182-7-dave@stgolabs.net
[ Added some missing conversions to rb_first_cached() ]
Signed-off-by: NArnaldo Carvalho de Melo <acme@redhat.com>