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.

Fixes: 8bc5482 ("iommu/amd: Convert from atomic_t to refcount_t on pasid_state->count")
Signed-off-by: Daniel Marcovitch <dmarcovitch@nvidia.com>
Signed-off-by: Vasant Hegde <vasant.hegde@amd.com>
Link: https://lore.kernel.org/r/20230609105146.7773-2-vasant.hegde@amd.com
Signed-off-by: Joerg Roedel <jroedel@suse.de>

Author: Daniel Marcovitch

drivers/iommu/amd/iommu_v2.c

@@ -262,8 +262,8 @@ static void put_pasid_state(struct pasid_state *pasid_state)
 
 static void put_pasid_state_wait(struct pasid_state *pasid_state)
 {
-	refcount_dec(&pasid_state->count);
-	wait_event(pasid_state->wq, !refcount_read(&pasid_state->count));
+	if (!refcount_dec_and_test(&pasid_state->count))
+		wait_event(pasid_state->wq, !refcount_read(&pasid_state->count));
 	free_pasid_state(pasid_state);
 }