dmaengine: idxd: Fix potential null dereference on pointer status

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

dmaengine: idxd: Fix potential null dereference on pointer status

There are calls to idxd_cmd_exec that pass a null status pointer however

a recent commit has added an assignment to *status that can end up

with a null pointer dereference. The function expects a null status

pointer sometimes as there is a later assignment to *status where

status is first null checked. Fix the issue by null checking status

before making the assignment.

Addresses-Coverity: ("Explicit null dereferenced")