iommu/amd: Fix incorrect error handling in amd_iommu_bind_pasid()

In function amd_iommu_bind_pasid(), the control flow jumps
to label out_free when pasid_state->mm and mm is NULL. And
mmput(mm) is called.  In function mmput(mm), mm is
referenced without validation. This will result in a NULL
dereference bug. This patch fixes the bug.
Signed-off-by: Pan Bian <bianpan2016@163.com>
Fixes: f0aac63b ('iommu/amd: Don't hold a reference to mm_struct')
Signed-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/amd_iommu_v2.c

@@ -696,9 +696,9 @@ int amd_iommu_bind_pasid(struct pci_dev *pdev, int pasid,
 
 out_unregister:
 	mmu_notifier_unregister(&pasid_state->mn, mm);
+	mmput(mm);
 
 out_free:
-	mmput(mm);
 	free_pasid_state(pasid_state);
 
 out: