KVM: x86/mmu: Add comment on try_cmpxchg64 usage in tdp_mmu_set_spte_atomic

Commit aee98a68 ("KVM: x86/mmu: Use try_cmpxchg64 in
tdp_mmu_set_spte_atomic") removed the comment that iter->old_spte is
updated when different logical CPU modifies the page table entry.
Although this is what try_cmpxchg does implicitly, it won't hurt
if this fact is explicitly mentioned in a restored comment.

Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: David Matlack <dmatlack@google.com>
Signed-off-by: Uros Bizjak <ubizjak@gmail.com>
Link: https://lore.kernel.org/r/20230425113932.3148-1-ubizjak@gmail.com


[sean: extend comment above try_cmpxchg64()]
Signed-off-by: Sean Christopherson <seanjc@google.com>