RDMA/core: Remove context entries from list while unregistering device

While unregistering a device, remove the context elements from the list to
not have any stale entries. With that any errors/bugs can be checked when
device is freed.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Daniel Jurgens <danielj@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>