CVE-2026-74355: iommu/vt-d: Fix RB-tree corruption in probe error path

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

iommu/vt-d: Fix RB-tree corruption in probe error path

The info->node RB-tree member is zero-initialized via kzalloc. If a device does not support ATS, the devicerbtreeinsert() call is skipped. If a subsequent probe step fails, the error path jumps to devicerbtreeremove(), which misinterprets the zeroed node as a tree root and corrupts the device RB-tree.

Fix this by explicitly initializing the RB-node as empty using RBCLEARNODE() during initialization and guarding the removal with RBEMPTYNODE().