crush: crush_init_workspace starts with struct crush_work

It is not just a pointer to crush_work, it is the whole structure.
That is not a problem since it only contains a pointer. But it will
be a problem if new data members are added to crush_work.

Reflects ceph.git commit ee957dd431bfbeb6dadaf77764db8e0757417328.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>