erofs: unify lcn as u64 for 32-bit platforms

In the Linux kernel, the following vulnerability has been resolved:

erofs: unify lcn as u64 for 32-bit platforms

As sashiko reported [1], lcn was typed as unsigned long (or unsigned int sometimes), which is only 32 bits wide on 32-bit platforms, which causes (lcn << lclusterbits) to be truncated at 4 GiB.

In order to consolidate the logic, just use u64 consistently around the codebase.

[1] https://sashiko.dev/r/20260420034612.1899973-1-hsiangkao%40linux.alibaba.com