RDMA/core: Use the ops infrastructure to keep all callbacks in one place

As preparation to hide rdma_restrack_root, refactor the code to use the
ops structure instead of a special callback which is hidden in
rdma_restrack_root.

Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>