jffs2: fix up rb_root initializations to use RB_ROOT

jffs2 uses rb_node = NULL; to zero rb_root.

The problem with this is that 17d9ddc72fb8bba0d4f678 ("rbtree: Add
support for augmented rbtrees") in the linux-next tree adds a new field
to that struct which needs to be NULL as well.  This patch uses RB_ROOT
as the intializer so all of the relevant fields will be NULL'd.

Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com>
Cc: Eric Paris <eparis@redhat.com>
Acked-by: David Woodhouse <dwmw2@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>