crypto: qat - flush misc workqueue during device shutdown

commit 3d4df408 upstream.

Repeated loading and unloading of a device specific QAT driver, for
example qat_4xxx, in a tight loop can lead to a crash due to a
use-after-free scenario. This occurs when a power management (PM)
interrupt triggers just before the device-specific driver (e.g.,
qat_4xxx.ko) is unloaded, while the core driver (intel_qat.ko) remains
loaded.

Since the driver uses a shared workqueue (`qat_misc_wq`) across all
devices and owned by intel_qat.ko, a deferred routine from the
device-specific driver may still be pending in the queue. If this
routine executes after the driver is unloaded, it can dereference freed
memory, resulting in a page fault and kernel crash like the following:

    BUG: unable to handle page fault for address: ffa000002e50a01c
    #PF: supervisor read access in kernel mode
    RIP: 0010:pm_bh_handler+0x1d2/0x250 [intel_qat]
    Call Trace:
      pm_bh_handler+0x1d2/0x250 [intel_qat]
      process_one_work+0x171/0x340
      worker_thread+0x277/0x3a0
      kthread+0xf0/0x120
      ret_from_fork+0x2d/0x50

To prevent this, flush the misc workqueue during device shutdown to
ensure that all pending work items are completed before the driver is
unloaded.

Note: This approach may slightly increase shutdown latency if the
workqueue contains jobs from other devices, but it ensures correctness
and stability.

Fixes: e5745f34 ("crypto: qat - enable power management for QAT GEN4")
Signed-off-by: Giovanni Cabiddu <giovanni.cabiddu@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Ahsan Atta <ahsan.atta@intel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>