RDMA/core: Always release restrack object

Change location of rdma_restrack_del() to fix the bug where
task_struct was acquired but not released, causing to resource leak.

  ucma_create_id() {
    ucma_alloc_ctx();
    rdma_create_user_id() {
      rdma_restrack_new();
      rdma_restrack_set_name() {
        rdma_restrack_attach_task.part.0(); <--- task_struct was gotten
      }
    }
    ucma_destroy_private_ctx() {
      ucma_put_ctx();
      rdma_destroy_id() {
        _destroy_id()                       <--- id_priv was freed
      }
    }
  }

Fixes: 889d916b

 ("RDMA/core: Don't access cm_id after its destruction")
Link: https://lore.kernel.org/r/073ec27acb943ca8b6961663c47c5abe78a5c8cc.1624948948.git.leonro@nvidia.com
Reported-by: Pavel Skripkin <paskripkin@gmail.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>