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.