iommu/vt-d: Check for allocation failure in aux_detach_device()

In current kernels small allocations never fail, but checking for
allocation failure is the correct thing to do.

Fixes: 18abda7a ("iommu/vt-d: Fix general protection fault in aux_detach_device()")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Lu Baolu <baolu.lu@linux.intel.com>
Link: https://lore.kernel.org/r/YJuobKuSn81dOPLd@mwanda
Link: https://lore.kernel.org/r/20210519015027.108468-2-baolu.lu@linux.intel.com


Signed-off-by: Joerg Roedel <jroedel@suse.de>