iommu/vt-d: Avoid NULL pointer dereference or refcount corruption

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

iommu/vt-d: Avoid NULL pointer dereference or refcount corruption

Commit 60f030f7418d ("iommu/vt-d: Avoid use of NULL after WARN_ON_ONCE")
fixed a NULL pointer dereference in an unlikely situation partly.

If dev_pasid is not found in the dev_pasids list, it remains NULL.
However, the teardown operations are executed unconditionally, this lead
to a NULL pointer dereference or refcount corruption.

If the domain was never attached to this IOMMU, info will be NULL, which
would cause an immediate dereference when checking --info->refcnt.

Even if info is not NULL, decrementing the refcount without having removed
a valid PASID might unbalance the count. This could lead to premature
dropping of the refcount to 0, potentially causing a use-after-free for the
remaining active devices sharing the domain.

Fix it by returning early if dev_pasid is NULL, before executing the
teardown operations.

Issue found by AI review and suggested by Kevin Tian.
https://sashiko.dev/#/patchset/20260421031347.1408890-1-zhenzhong.duan%40intel.com