Make the public API clearer - if the user wants to allocate a
spdk_copy_task directly, they need to allocate spdk_copy_task_size()
bytes.
Also change the return type to size_t for consistency.
Change-Id: I0f3757056757c510421d680c5b4532edd9bc2561
Signed-off-by: Daniel Verkamp <daniel.verkamp@intel.com>