KVM: x86/mmu: Bug the VM if KVM tries to split a !hugepage SPTE

Bug the VM instead of simply warning if KVM tries to split a SPTE that is
non-present or not-huge.  KVM is guaranteed to end up in a broken state as
the callers fully expect a valid SPTE, e.g. the shadow MMU will add an
rmap entry, and all MMUs will account the expected small page.  Returning
'0' is also technically wrong now that SHADOW_NONPRESENT_VALUE exists,
i.e. would cause KVM to create a potential #VE SPTE.

While it would be possible to have the callers gracefully handle failure,
doing so would provide no practical value as the scenario really should be
impossible, while the error handling would add a non-trivial amount of
noise.

Fixes: a3fe5dbd ("KVM: x86/mmu: Split huge pages mapped by the TDP MMU when dirty logging is enabled")
Cc: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-ID: <20240712151335.1242633-2-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>