dmaengine: idxd: Prevent use after free on completion memory

In the Linux kernel, the following vulnerability has been resolved:

dmaengine: idxd: Prevent use after free on completion memory

On driver unload any pending descriptors are flushed at the
time the interrupt is freed:
idxd_dmaengine_drv_remove() ->
drv_disable_wq() ->
idxd_wq_free_irq() ->
idxd_flush_pending_descs().

If there are any descriptors present that need to be flushed this
flow triggers a "not present" page fault as below:

BUG: unable to handle page fault for address: ff391c97c70c9040
#PF: supervisor read access in kernel mode
#PF: error_code(0x0000) - not-present page

The address that triggers the fault is the address of the
descriptor that was freed moments earlier via:
drv_disable_wq()->idxd_wq_free_resources()

Fix the use after free by freeing the descriptors after any possible
usage. This is done after idxd_wq_reset() to ensure that the memory
remains accessible during possible completion writes by the device.