KVM: arm64: Drop last page ref in kvm_pgtable_stage2_free_removed()

The reference count on page table allocations is increased for every
'counted' PTE (valid or donated) in the table in addition to the initial
reference from ->zalloc_page(). kvm_pgtable_stage2_free_removed() fails
to drop the last reference on the root of the table walk, meaning we
leak memory.

Fix it by dropping the last reference after the free walker returns,
at which point all references for 'counted' PTEs have been released.

Cc: [email protected]
Fixes: 5c359cc ("KVM: arm64: Tear down unlinked stage-2 subtree after break-before-make")
Reported-by: Yu Zhao <[email protected]>
Signed-off-by: Oliver Upton <[email protected]>
Tested-by: Yu Zhao <[email protected]>
Signed-off-by: Marc Zyngier <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

Author: oupton

arch/arm64/kvm/hyp/pgtable.c

@@ -1332,4 +1332,7 @@ void kvm_pgtable_stage2_free_removed(struct kvm_pgtable_mm_ops *mm_ops, void *pg
 	};
 
 	WARN_ON(__kvm_pgtable_walk(&data, mm_ops, ptep, level + 1));
+
+	WARN_ON(mm_ops->page_count(pgtable) != 1);
+	mm_ops->put_page(pgtable);
 }