IPoIB: Make sure no receives are handled when stopping device

The current IPoIB code might process receive completions from
ipoib_drain_cq() when bringing down the interface.  This could cause
packets to be passed up the stack without the device's poll method
being called.  Avoid this by setting the status of any successful
completions to IB_WC_WR_FLUSH_ERR.

Signed-off-by: Roland Dreier <rolandd@cisco.com>