dmaengine: idxd: Check availability of workqueue allocated by idxd wq driver before using

Running IDXD workloads in a container with the /dev directory mounted can
trigger a call trace or even a kernel panic when the parent process of the
container is terminated.

This issue occurs because, under certain configurations, Docker does not
properly propagate the mount replica back to the original mount point.

In this case, when the user driver detaches, the WQ is destroyed but it
still calls destroy_workqueue() attempting to completes all pending work.
It's necessary to check wq->wq and skip the drain if it no longer exists.

Signed-off-by: Yi Sun <yi.sun@intel.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>

Link: https://lore.kernel.org/r/20250509000304.1402863-1-yi.sun@intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>