ext3 uses rb_node = NULL; to zero rb_root.

The problem with this is that 17d9ddc7 ("rbtree: Add support
for augmented rbtrees") in the linux-next tree adds a new field to that
struct which needs to be NULLas 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>
Cc: Jan Kara <jack@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jan Kara <jack@suse.cz>