staging: erofs: complete error handing of z_erofs_do_read_page

This patch completes error handing code of z_erofs_do_read_page.
PG_error will be set when some read error happens, therefore
z_erofs_onlinepage_endio will unlock this page without setting
PG_uptodate.

Reviewed-by: Chao Yu <yucxhao0@huawei.com>
Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>