Currently we may have work scheduled in default kernel workqueue when
the device is going down.  The device could get freed before this
workqueue gets serviced.  I am actually seeing this causing system
hangs.

The following patch fixes this by using ipoib_workqueue which gets
flushed when the device is going down.
Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>