tracing/stat: replace linked list by an rbtree for sorting

When the stat tracing framework prepares the entries from a tracer
to output them to the user, it starts by computing a linear sort
through a linked list to give the entries ordered by relevance
to the user.

This is quite ugly and causes a small latency when we begin to
read the file.

This patch changes that by turning the linked list into a red-black
tree. Athough the whole iteration using the start and next tracer
callbacks while opening the file remain the same, it is now much
more fast and scalable.

The rbtree guarantees O(log(n)) insertions whereas a linked
list with linear sorting brought us a O(n) despair. Now the
(visible) latency has disapeared.

[ Impact: kill the latency while starting to read a stat tracer file ]

Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>