RDMA/cxgb3: Don't complete flushed send work requests twice

When the SQ is flushed, mark the flushed entries as not signaled so
the poll logic doesn't re-insert the CQ entry thinking its an out of
order completion.

The bug can cause the NFS/RDMA server to crash due to processing the
same completed work request twice.

Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>