IB/cm: Do not queue work to a device that's going away

IB/cm: Do not queue work to a device that's going away

Whenever ib_cm gets remove_one call, like when there is a hot-unplug
event, the driver should mark itself as going_down and confirm that no
new works are going to be queued for that device.
so, the order of the actions are:
1. mark the going_down bit.
2. flush the wq.
3. [make sure no new works for that device.]
4. unregister mad agent.

otherwise, works that are already queued can be scheduled after the mad
agent was freed.

Signed-off-by: Erez Shitrit <erezsh@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>