tools lib rbtree: Pick some improvements from the kernel rbtree code

tools lib rbtree: Pick some improvements from the kernel rbtree code

The tools/lib/rbtree.c code came from the kernel, removing the
EXPORT_SYMBOL() that make sense only there, unfortunately it is not
being checked with tools/perf/check_headers.sh, will try to remedy this,
till then pick the improvements from:

  b0687c1119b4e8c8 ("lib/rbtree: use '+' instead of '|' for setting color.")

That I noticed by doing:

  diff -u tools/lib/rbtree.c lib/rbtree.c
  diff -u tools/include/linux/rbtree_augmented.h include/linux/rbtree_augmented.h

There is one other cases, but lets pick it in separate patches.

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Kan Liang <kan.liang@linux.intel.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Noah Goldstein <goldstein.w.n@gmail.com>
Link: https://lore.kernel.org/lkml/ZigZzeFoukzRKG1Q@x1
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>