KVM: arm64: nv: Avoid dereferencing NULL VNCR pseudo-TLB

In the Linux kernel, the following vulnerability has been resolved: KVM: arm64: nv: Avoid dereferencing NULL VNCR pseudo-TLB VNCR TLB invalidation occurs from MMU notifiers or TLBI instructions, and either can race against a vcpu not being onlined yet (no pseudo-TLB allocated). Similarly, the TLB might be invalid, and the invalidation should be skipped in this case. Both kvm_invalidate_vncr_ipa() and kvm_invalidate_vncr_va() are expected to perform the same checks, except that the latter doesn't check for the allocation and blindly dereferences the pointer. Solve this by introducing a new iterator built on top of the usual kvm_for_each_vcpu() that checks for both of the above conditions, and convert the two users to it.