KVM: arm64: nv: Fix life cycle of the nested_mmus array

In the Linux kernel, the following vulnerability has been resolved:

KVM: arm64: nv: Fix life cycle of the nested_mmus array

The nested_mmus array holds the shadow page tables that are used when
a guest is running a nested context. These structures are allocated on
VCPU_INIT for whole guest, which implies that they may have to be
relocated as the array grows.

Should a VCPU_INIT occur whilst a vcpu is actively running an L2 and
that the allocation requires relocation, that vcpu will still be
running with a pointer to the previous structure, which will have been
freed.

Fix this by turning the array of structures to an array of pointers,
which is now allocated at VM creation, sized to the absolute maximum
that KVM can handle.

In turn, each VCPU_INIT contributes S2_MMU_PER_VCPU to the pool. No
reallocation is ever performed, and the life cycle of each object is
much clearer:

- the nested_mmus array is allocated in kvm_init_nested(), and freed
in kvm_arch_destroy_vm()

- s2_mmu structures are allocated in kvm_vcpu_init_nested(), and freed
on kvm_arch_flush_shadow_all()

Finally, the freeing of vcpu->arch.vncr_array is made consistent
rather than being done on some failure paths, but not others.