[malloc] Avoid immediately clobbering reference count when freeing memory

[malloc] Avoid immediately clobbering reference count when freeing memory

Rearrange the fields in struct memory_block (without altering
MIN_MEMBLOCK_SIZE) so that the "count" field of a reference-counted
object is left intact when the memory containing the object is freed.
This allows for the possibility of detecting reference-counting errors
such as double-freeing.

Signed-off-by: Michael Brown <mcb30@ipxe.org>