This patch modifies @struct ubi_wl_entry and adds union which
contains only one element so far. This is just a preparation
for further changes which will kill the protection tree and
make UBI use a list instead.
Signed-off-by: NXiaochuan-Xu <xiaochuan-xu@cqu.edu.cn>
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>