Function safe_app::ffi::nfs::file_write

#[no_mangle]
pub unsafe extern "C" fn file_write(
    app: *const App, 
    file_h: FileContextHandle, 
    data: *const u8, 
    data_len: usize, 
    user_data: *mut c_void, 
    o_cb: extern "C" fn(user_data: *mut c_void, result: *const FfiResult)
)

Write data to file in smaller chunks.