staging: erofs: fix integer overflow on 32-bit platform

This patch fixes integer overflow on multiplication
of 32-bit `lcn' in z_erofs_map_blocks_iter.

Signed-off-by: Gao Xiang <gaoxiang25@huawei.com>
Reviewed-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Chao Yu <yuchao0@huawei.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>