RDMA/nes: Clean out CQ completions when QP is destroyed

When a QP is destroyed, unprocessed CQ entries could still reference
the QP.  This change zeroes the context value at QP destroy time.  By
skipping over cqe's with a zero context, poll_cq no longer processes a
cqe for a destroyed QP.

Signed-off-by: Don Wood <donald.e.wood@intel.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>