IB/sa_query: Flush scheduled work before unloading module

sa_query schedules work on IB asynchronous events.  After
unregistering the async event handler, make sure that this work has
completed before releasing the IB device (and possibly allowing the
sa_query module text to go away).

Signed-off-by: Michael S. Tsirkin <mst@mellanox.co.il>
Signed-off-by: Roland Dreier <rolandd@cisco.com>