Maybe unsound in free_mutex

[lwz23]

Hello, thank you for your contribution in this project, I am scanning the unsoundness problem in rust project.
I notice the following code:

pub fn free_mutex(mutex: *mut KMutex) {
        let index = unsafe { mutex.offset_from(&rust_mutex_pool[0] as *const _) } as usize;
        let byte = index / 8;
        let bit = index % 8;
        USED[byte].fetch_and(!(1 << bit), Ordering::Relaxed);
    }

Considering that pub mod mutex_alloc, and free_mutex is also a pub function. I assume that users can directly call this function. This potential situation could result in unsafe { mutex.offset_from(&rust_mutex_pool[0] as *const _) } being operating on a null pointer, I guess it might trigger undefined behavior (UB). For safety reasons, I felt it necessary to report this issue. If you have performed checks elsewhere that ensure this is safe, please don’t take offense at my raising this issue.
I suggest Several possible fixes:

1. If there is no external usage for free_mutex, it should not marked as pub.
2. free_mutex method should add additional check for null pointer.
3. mark free_mutex method as unsafe and proper doc to let users know that they should provide valid Pointers.

[lwz23]

ping?