erofs: directly use wrapper erofs_page_is_managed() when shrinking

We already have the wrapper function to identify managed page.

Link: https://lore.kernel.org/r/20210810065450.1320-1-zbestahu@gmail.com
Reviewed-by: Gao Xiang <hsiangkao@linux.alibaba.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Yue Hu <huyue2@yulong.com>
Signed-off-by: Gao Xiang <hsiangkao@linux.alibaba.com>