util/rb_tree: Reverse the order of comparison functions

The new order matches that of the comparison functions accepted by the C
standard library qsort() functions.  Being consistent with qsort will
hopefully help avoid developer confusion.

The only current user of the red-black tree is aub_mem.c which is pretty
easy to fix up.

Reviewed-by: Lionel Landwerlin <lionel.g.lndwerlin@intel.com>
40 jobs for !2058 with review/rb-tree-reverse in 19 minutes and 38 seconds (queued for 5 seconds)
merge request