[JFFS2] Optimise jffs2_add_tn_to_list

Use an rbtree instead of a simple linked list. We were wasting 
an amazing amount of time in jffs2_add_tn_to_list(). 
Thanks to Artem Bityuckiy and Jarkko Jlavinen  for noticing.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>