KVM: x86/mmu: Bug the VM if KVM attempts to unsync an upper-level shadow page

Bug the VM and skip marking a shadow page unsync if KVM attempts to unsync
an upper-level shadow page, as marking an upper-level SP unsync is all but
guaranteed to be far worse than terminating the VM.  E.g. sync_spte()
(rightly) assumes SPTEs in the SP are leaf SPTEs and so calls drop_spte()
instead of drop_parent_pte(), which can effectively crash the host via
when running with CONFIG_BUG_ON_DATA_CORRUPTION=y due to there not being an
rmap entry for the SPTE.

Simply not marking the SP unsync, i.e. letting the VM continue on, is
equally dangerous for the guest as it means KVM's shadow MMU is using stale
information, e.g. could unintentionally corrupt the guest.

Cc: Jinu Kim <kimjw04271234@gmail.com>
Link: https://patch.msgid.link/20260806191945.4192732-1-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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