KVM: x86/mmu: WARN on an invalid root_hpa

WARN on the existing invalid root_hpa checks in __direct_map() and
FNAME(fetch).  The "legitimate" path that invalidated root_hpa in the
middle of a page fault is long since gone, i.e. it should no longer be
impossible to invalidate in the middle of a page fault[*].

The root_hpa checks were added by two related commits

  989c6b34 ("KVM: MMU: handle invalid root_hpa at __direct_map")
  37f6a4e2 ("KVM: x86: handle invalid root_hpa everywhere")

to fix a bug where nested_vmx_vmexit() could be called *in the middle*
of a page fault.  At the time, vmx_interrupt_allowed(), which was and
still is used by kvm_can_do_async_pf() via ->interrupt_allowed(),
directly invoked nested_vmx_vmexit() to switch from L2 to L1 to emulate
a VM-Exit on a pending interrupt.  Emulating the nested VM-Exit resulted
in root_hpa being invalidated by kvm_mmu_reset_context() without
explicitly terminating the page fault.

Now that root_hpa is checked for validity by kvm_mmu_page_fault(), WARN
on an invalid root_hpa to detect any flows that reset the MMU while
handling a page fault.  The broken vmx_interrupt_allowed() behavior has
long since been fixed and resetting the MMU during a page fault should
not be considered legal behavior.

[*] It's actually technically possible in FNAME(page_fault)() because it
    calls inject_page_fault() when the guest translation is invalid, but
    in that case the page fault handling is immediately terminated.

Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>