KVM: arm64: Mark freed S2 MMUs as invalid

When freeing an S2 MMU, we free the associated pgd, but omit to
mark the structure as invalid. Subsequently, a call to
kvm_nested_s2_unmap() would pick these invalid S2 MMUs and
pass them down the teardown path.

This ends up with a nasty warning as we try to unmap an unallocated
set of page tables.

Fix this by making the S2 MMU invalid on freeing the pgd by calling
kvm_init_nested_s2_mmu().

Fixes: 4f128f8 ("KVM: arm64: nv: Support multiple nested Stage-2 mmu structures")
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20250905072859.211369-1-maz@kernel.org
Signed-off-by: Oliver Upton <oliver.upton@linux.dev>

Author: Marc Zyngier

arch/arm64/kvm/mmu.c

@@ -1106,6 +1106,10 @@ void kvm_free_stage2_pgd(struct kvm_s2_mmu *mmu)
 		mmu->pgt = NULL;
 		free_percpu(mmu->last_vcpu_ran);
 	}
+
+	if (kvm_is_nested_s2_mmu(kvm, mmu))
+		kvm_init_nested_s2_mmu(mmu);
+
 	write_unlock(&kvm->mmu_lock);
 
 	if (pgt) {