KVM: x86/mmu: Warn if PFN changes on shadow-present SPTE in shadow MMU

KVM: x86/mmu: Warn if PFN changes on shadow-present SPTE in shadow MMU

Warn if PFN changes on shadow-present SPTE in mmu_set_spte().

KVM should _never_ change the PFN of a shadow-present SPTE. In
mmu_set_spte(), there is a WARN_ON_ONCE() on pfn changes on shadow-present
SPTE in mmu_spte_update() to detect this condition. However, that
WARN_ON_ONCE() is not hittable since mmu_set_spte() invokes drop_spte()
earlier before mmu_spte_update(), which clears SPTE to a !shadow-present
state. So, before invoking drop_spte(), add a WARN_ON_ONCE() in
mmu_set_spte() to warn PFN change of a shadow-present SPTE.

For the spurious prefetch fault, only return RET_PF_SPURIOUS directly when
PFN is not changed. When PFN changes, fall through to follow the sequence
of drop_spte(), warn of PFN change, make_spte(), flush tlb, rmap_add().

Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://lore.kernel.org/r/20250318013310.5781-1-yan.y.zhao@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>