Invalid .as_ptr() type casts on some arm platforms

[maxwnewcomer]

Describe the bug
When compiling mayastor with an arm compatible version of spdk-rs (openebs/spdk-rs#72), mayastor still fails to build due to invalid to_ptr() typecasts, one example shown here:

ftl_conf.name = ftl_dev_name.as_ptr() as *mut i8;

On some platforms, as noted in the CString docs, are actually represented by u8 and not i8 resulting in

error[E0308]: mismatched types
   --> io-engine/src/bdev/ftl.rs:213:25
    |
213 |         ftl_conf.name = ftl_dev_name.as_ptr() as *mut i8;
    |         -------------   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ expected `*mut u8`, found `*mut i8`
    |         |
    |         expected due to the type of this binding
    |
    = note: expected raw pointer `*mut u8`
               found raw pointer `*mut i8`

in multiple places (including libnvme-rs + io-engine compilation)

To Reproduce

1. Access a linux/aarch64 machine
2. Pull develop mayastor branch
3. Re-point spdk-rs submodule to fork at above linked issue
4. nix-shell
5. cargo build

Expected behavior

Full compilation!

OS info (please complete the following information):

Distributor ID: Ubuntu
Description: Ubuntu 24.04.1 LTS
Release: 24.04

Mayastor: 687da6c