RDMA/rdma_cm: Make rdma_addr_cancel into a fence

Currently rdma_addr_cancel does not prevent the callback from being used,
this is surprising and hard to reason about. There does not appear to be a
bug here as the only user of this API does refcount properly, fixing it
only to increase clarity.

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