erofs: remove the mapping parameter from erofs_try_to_free_cached_page()

The mapping is not used at all, remove it and update related code.

Link: https://lore.kernel.org/r/20210810072416.1392-1-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>