These were added to deal with duplicated functionality within btrfs-progs, but
we specifically copied rbtree.c from the kernel, so move these functions out
into their own file. This will make it easier to keep rbtree.c in sync. Thanks,
Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
