rslib: Several improvements

[Zhang Boyang]

Hello,

I made several improvements to reed-solomon library.
Please have a look. :)

Thanks!

Best Regards,
Zhang Boyang

===

Changes in [PATCH v2]:
Added more patches. Removed init_rs16(), since init_rs_non_canonical()
can do the same job.

Changes in [PATCH v1]:
Added init_rs16().
Link: https://lore.kernel.org/all/20220606101901.83538-1-zhangboyang.id@xxxxxxxxx/

[RFC PATCH]:
Link: https://lore.kernel.org/all/20220605073857.126497-1-zhangboyang.id@xxxxxxxxx/