Migrate transmute::FromBytes to zerocopy's #1241

We are landing zerocopy in the kernel tree, thus migrate transmute::FromBytes to zerocopy's wherever possible.

Also consider #975.

This requires submitting a proper patch to the LKML and the Rust for Linux mailing list. Please recall to test your changes (including generating the documentation if changed, running the Rust doctests if changed, etc.), to justify the commit well in the message ("what" & "why"), to use a proper title for the commit, to sign your commit under the Developer's Certificate of Origin and to add a Suggested-by: tag, and a Link: tag to this issue. Please see https://docs.kernel.org/process/submitting-patches.html and https://rust-for-linux.com/contributing for details.