Fix BlockBitSet for ThreadSafeBlockBitSet bitset vector calculation
`ptr` is already a `*mut Self`, so `offset` is in units of
`core::mem::size_of<Self>` and so only need `1` to jump past `Self` to
bitvector.  Using `core::mem::size_of<Self>` would only make sense it
`ptr` was `*_ u8`.
@@ -98,7 +98,7 @@ impl BlockBitSet for ThreadSafeBlockBitSet {
let num_blocks = calculate_block_fit::<Self>(size, block_size);
let extent_layout = Self::extended_layout(num_blocks);
// Calculate pointer to beginning of bit vector
- let vector = ptr.offset(mem::size_of::<Self>() as isize) as *mut AtomicUsize;
+ let vector = ptr.offset(1) as *mut AtomicUsize;
// Write initial state to bit vector, using calculated pointer
let num_elems = units_required_for::<Self>(num_blocks);
for i in 0..num_elems {
