cxgb3: fix workqueue flush issues

The fatal error task can be scheduled while processing an offload packet
in NAPI context when the connection handle is bogus. this can race
with the ports being brought down and the cxgb3 workqueue being flushed.
Stop napi processing before flushing the work queue.

The ULP drivers (iSCSI, iWARP) might also schedule a task on keventd_wk
while releasing a connection handle (cxgb3_offload.c::cxgb3_queue_tid_release()).
The driver however does not flush any work on keventd_wq while being unloaded.
This patch also fixes this.

Also call cancel_delayed_work_sync in place of the the deprecated
cancel_rearming_delayed_workqueue.

Signed-off-by: Divy Le Ray <divy@chelsio.com>
Signed-off-by: David S. Miller <davem@davemloft.net>