iommu/amd/iommu_v2: Fix pasid_state refcount dec hit 0 warning on pasid unbind

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

iommu/amd/iommu_v2: Fix pasid_state refcount dec hit 0 warning on pasid unbind

When unbinding pasid - a race condition exists vs outstanding page faults.

To prevent this, the pasid_state object contains a refcount.

* set to 1 on pasid bind

* incremented on each ppr notification start

* decremented on each ppr notification done

* decremented on pasid unbind

Since refcount_dec assumes that refcount will never reach 0:

the current implementation causes the following to be invoked on

pasid unbind:

REFCOUNT_WARN("decrement hit 0; leaking memory")

Fix this issue by changing refcount_dec to refcount_dec_and_test

to explicitly handle refcount=1.