lib/rbtree_test.c: make input module parameters

Allows for more flexible debugging.

Link: http://lkml.kernel.org/r/20170719014603.19029-5-dave@stgolabs.net
Signed-off-by: Davidlohr Bueso <dbueso@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>