KVM: x86/tdp_mmu: WARN if PFN changes for spurious faults

KVM: x86/tdp_mmu: WARN if PFN changes for spurious faults

Add a WARN() to assert that KVM does _not_ change the PFN of a
shadow-present SPTE during spurious fault handling.

KVM should _never_ change the PFN of a shadow-present SPTE and TDP MMU
already BUG()s on this. However, spurious faults just return early before
the existing BUG() could be hit.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Yan Zhao <yan.y.zhao@intel.com>
Link: https://lore.kernel.org/r/20250318013238.5732-1-yan.y.zhao@intel.com
Signed-off-by: Sean Christopherson <seanjc@google.com>