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")