Fix compilation errors on platforms with unsigned c_char

[refi64]

The docs for c_char mention:

On modern architectures this type will always be either i8 or u8

It's u8 specifically on multiple prominent architectures, such as arm64 and riscv:

https://github.com/rust-lang/rust/blob/f29256dd1420dc681bf4956e3012ffe9eccdc7e7/library/core/src/ffi/primitives.rs#L42

Thus, this updates the places where c_char is intermixed with i8 to exclusively rely on the former.

[refi64]

I must note that I'm not really familiar with btrfs and thus didn't really test this past compilation. I expect it should work, because nothing really major changed, but still! This project is just a dependency of something else I had to build on an arm64 host system.