kvm: fix usage of uninit spinlock in avic_vm_destroy()

If avic is not enabled, avic_vm_init() does nothing and returns early.
However, avic_vm_destroy() still tries to destroy what hasn't been created.
The only bad consequence of this now is that avic_vm_destroy() uses
svm_vm_data_hash_lock that hasn't been initialized (and is not meant
to be used at all if avic is not enabled).

Return early from avic_vm_destroy() if avic is not enabled.
It has nothing to destroy.

Signed-off-by: Dmitry Vyukov <dvyukov@google.com>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: "Radim Krčmář" <rkrcmar@redhat.com>
Cc: David Hildenbrand <david@redhat.com>
Cc: kvm@vger.kernel.org
Cc: syzkaller@googlegroups.com
Reviewed-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Radim Krčmář <rkrcmar@redhat.com>