dmaengine: idxd: Fix possible invalid memory access after FLR

dmaengine: idxd: Fix possible invalid memory access after FLR

In the case that the first Function Level Reset (FLR) concludes
correctly, but in the second FLR the scratch area for the saved
configuration cannot be allocated, it's possible for a invalid memory
access to happen.

Always set the deallocated scratch area to NULL after FLR completes.

Fixes: 98d187a98903 ("dmaengine: idxd: Enable Function Level Reset (FLR) for halt")
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>
Link: https://patch.msgid.link/20260121-idxd-fix-flr-on-kernel-queues-v3-v3-3-7ed70658a9d1@intel.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>