KVM: x86/mmu: Don't overwrite shadow-present MMU SPTEs when prefaulting

Treat attempts to prefetch/prefault MMU SPTEs as spurious if there's an
existing shadow-present SPTE, as overwriting a SPTE that may have been
create by a "real" fault is at best confusing, and at worst potentially
harmful.  E.g. mmu_try_to_unsync_pages() doesn't unsync when prefetching,
which creates a scenario where KVM could try to replace a Writable SPTE
with a !Writable SPTE, as sp->unsync is checked prior to acquiring
mmu_unsync_pages_lock.

Note, this applies to three of the four flavors of "prefetch" in KVM:

  - KVM_PRE_FAULT_MEMORY
  - Async #PF (host or PV)
  - Prefetching

The fourth flavor, SPTE synchronization, i.e. FNAME(sync_spte), _only_
overwrites shadow-present SPTEs when calling make_spte().  But SPTE
synchronization specifically uses mmu_spte_update(), and so naturally
avoids the @prefetch check in mmu_set_spte().

Signed-off-by: Sean Christopherson <seanjc@google.com>
Tested-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <20241010182427.1434605-6-seanjc@google.com>