KVM: PPC: Book3S HV: fix incorrect NULL check on list iterator

The bug is here:
	if (!p)
                return ret;

The list iterator value 'p' will *always* be set and non-NULL by
list_for_each_entry(), so it is incorrect to assume that the iterator
value will be NULL if the list is empty or no element is found.

To fix the bug, Use a new value 'iter' as the list iterator, while use
the old value 'p' as a dedicated variable to point to the found element.

Fixes: dfaa973a ("KVM: PPC: Book3S HV: In H_SVM_INIT_DONE, migrate remaining normal-GFNs to secure-GFNs")
Cc: stable@vger.kernel.org # v5.9+
Signed-off-by: Xiaomeng Tong <xiam0nd.tong@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220414062103.8153-1-xiam0nd.tong@gmail.com

arch/powerpc/kvm/book3s_hv_uvmem.c

@@ -361,13 +361,15 @@ static bool kvmppc_gfn_is_uvmem_pfn(unsigned long gfn, struct kvm *kvm,
 static bool kvmppc_next_nontransitioned_gfn(const struct kvm_memory_slot *memslot,
 		struct kvm *kvm, unsigned long *gfn)
 {
-	struct kvmppc_uvmem_slot *p;
+	struct kvmppc_uvmem_slot *p = NULL, *iter;
 	bool ret = false;
 	unsigned long i;
 
-	list_for_each_entry(p, &kvm->arch.uvmem_pfns, list)
-		if (*gfn >= p->base_pfn && *gfn < p->base_pfn + p->nr_pfns)
+	list_for_each_entry(iter, &kvm->arch.uvmem_pfns, list)
+		if (*gfn >= iter->base_pfn && *gfn < iter->base_pfn + iter->nr_pfns) {
+			p = iter;
 			break;
+		}
 	if (!p)
 		return ret;
 	/*