iommu/vt-d: Clean up si_domain in the init_dmars() error path

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

iommu/vt-d: Clean up si_domain in the init_dmars() error path

A splat from kmem_cache_destroy() was seen with a kernel prior to commit ee2653bbe89d ("iommu/vt-d: Remove domain and devinfo mempool") when there was a failure in init_dmars(), because the iommu_domain cache still had objects. While the mempool code is now gone, there still is a leak of the si_domain memory if init_dmars() fails. So clean up si_domain in the init_dmars() error path.