iw_cxgb4: when flushing, complete all wrs in a chain

If a wr chain was posted and needed to be flushed, only the first
wr in the chain was completed with FLUSHED status.  The rest were
never completed.  This caused isert to hang on shutdown due to the
missing completions which left iscsi IO commands referenced, stalling
the shutdown.

Fixes: 4fe7c296 ("iw_cxgb4: refactor sq/rq drain logic")

Cc: stable@vger.kernel.org
Signed-off-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>