KVM: arm64: Mark freed S2 MMUs as invalid

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: 4f128f8e1aaa ("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>