rbtree: changes to align the code with Linux tree

rbtree: changes to align the code with Linux tree

The patch aligns the code of rbtree related files with Linux tree.
This will minimize the conflicts during any future porting from Linux tree.

Linux commit till f4b477c47332367d35686bd2b808c2156b96d7c7 for rbtree.h
This includes addition of commented inline functions in rbtree.h, to have
complete replica from Linux tree.

Linux commit till 4c60117811171d867d4f27f17ea07d7419d45dae for rbtree.c
This includes updates in comments in header note in rbtree.c.

Signed-off-by: Praveen Kumar <kpraveen.lkml@gmail.com>
Acked-by: Jan Beulich <jbeulich@suse.com>