cnic: Fix use-after-free bugs in cnic_delete_task

cnic: Fix use-after-free bugs in cnic_delete_task

The original code uses cancel_delayed_work() in cnic_cm_stop_bnx2x_hw(),
which does not guarantee that the delayed work item 'delete_task' has
fully completed if it was already running. Additionally, the delayed work
item is cyclic, the flush_workqueue() in cnic_cm_stop_bnx2x_hw() only
blocks and waits for work items that were already queued to the
workqueue prior to its invocation. Any work items submitted after
flush_workqueue() is called are not included in the set of tasks that the
flush operation awaits. This means that after the cyclic work items have
finished executing, a delayed work item may still exist in the workqueue.
This leads to use-after-free scenarios where the cnic_dev is deallocated
by cnic_free_dev(), while delete_task remains active and attempt to
dereference cnic_dev in cnic_delete_task().

A typical race condition is illustrated below:

CPU 0 (cleanup)              | CPU 1 (delayed work callback)
cnic_netdev_event()          |
  cnic_stop_hw()             | cnic_delete_task()
    cnic_cm_stop_bnx2x_hw()  | ...
      cancel_delayed_work()  | /* the queue_delayed_work()
      flush_workqueue()      |    executes after flush_workqueue()*/
                             | queue_delayed_work()
  cnic_free_dev(dev)//free   | cnic_delete_task() //new instance
                             |   dev = cp->dev; //use

Replace cancel_delayed_work() with cancel_delayed_work_sync() to ensure
that the cyclic delayed work item is properly canceled and that any
ongoing execution of the work item completes before the cnic_dev is
deallocated. Furthermore, since cancel_delayed_work_sync() uses
__flush_work(work, true) to synchronously wait for any currently
executing instance of the work item to finish, the flush_workqueue()
becomes redundant and should be removed.

This bug was identified through static analysis. To reproduce the issue
and validate the fix, I simulated the cnic PCI device in QEMU and
introduced intentional delays — such as inserting calls to ssleep()
within the cnic_delete_task() function — to increase the likelihood
of triggering the bug.

Fixes: fdf24086f475 ("cnic: Defer iscsi connection cleanup")
Signed-off-by: Duoming Zhou <duoming@zju.edu.cn>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>