Fix mutable borrow from immutable input(s)

[kpp]

For further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mut_from_ref

Clippy errors:

error: mutable borrow from immutable input(s)
   --> ocl/src/async/buffer_sink.rs:359:42
    |
359 |     pub unsafe fn as_mut_slice(&self) -> &mut [T] {
    |        

note: immutable borrow here
   --> ocl/src/async/buffer_sink.rs:359:32
    |
359 |     pub unsafe fn as_mut_slice(&self) -> &mut [T] {
    |                                ^^^^^
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#mut_from_ref


error: mutable borrow from immutable input(s)
   --> ocl/src/async/buffer_stream.rs:371:42
    |
371 |     pub unsafe fn as_mut_slice(&self) -> &mut [T] {
    |                                          ^^^^^^^^
    |

error: mutable borrow from immutable input(s)
  --> ocl/src/async/rw_vec.rs:68:42
   |
68 |     pub unsafe fn as_mut_slice(&self) -> &mut [T] {
   |                                          ^^^^^^^^
   |