KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE

KVM: x86/mmu: Drop/zap existing present SPTE even when creating an MMIO SPTE

When installing an emulated MMIO SPTE, do so *after* dropping/zapping the
existing SPTE (if it's shadow-present).  While commit a54aa15c6bda3 was
right about it being impossible to convert a shadow-present SPTE to an
MMIO SPTE due to a _guest_ write, it failed to account for writes to guest
memory that are outside the scope of KVM.

E.g. if host userspace modifies a shadowed gPTE to switch from a memslot
to emulted MMIO and then the guest hits a relevant page fault, KVM will
install the MMIO SPTE without first zapping the shadow-present SPTE.

  ------------[ cut here ]------------
  is_shadow_present_pte(*sptep)
  WARNING: arch/x86/kvm/mmu/mmu.c:484 at mark_mmio_spte+0xb2/0xc0 [kvm], CPU#0: vmx_ept_stale_r/4292
  Modules linked in: kvm_intel kvm irqbypass
  CPU: 0 UID: 1000 PID: 4292 Comm: vmx_ept_stale_r Not tainted 7.0.0-rc2-eafebd2d2ab0-sink-vm #319 PREEMPT
  Hardware name: QEMU Standard PC (Q35 + ICH9, 2009), BIOS 0.0.0 02/06/2015
  RIP: 0010:mark_mmio_spte+0xb2/0xc0 [kvm]
  Call Trace:
   <TASK>
   mmu_set_spte+0x237/0x440 [kvm]
   ept_page_fault+0x535/0x7f0 [kvm]
   kvm_mmu_do_page_fault+0xee/0x1f0 [kvm]
   kvm_mmu_page_fault+0x8d/0x620 [kvm]
   vmx_handle_exit+0x18c/0x5a0 [kvm_intel]
   kvm_arch_vcpu_ioctl_run+0xc55/0x1c20 [kvm]
   kvm_vcpu_ioctl+0x2d5/0x980 [kvm]
   __x64_sys_ioctl+0x8a/0xd0
   do_syscall_64+0xb5/0x730
   entry_SYSCALL_64_after_hwframe+0x4b/0x53
  RIP: 0033:0x47fa3f
   </TASK>
  ---[ end trace 0000000000000000 ]---

Reported-by: Alexander Bulekov <bkov@amazon.com>
Debugged-by: Alexander Bulekov <bkov@amazon.com>
Suggested-by: Fred Griffoul <fgriffo@amazon.co.uk>
Fixes: a54aa15c6bda3 ("KVM: x86/mmu: Handle MMIO SPTEs directly in mmu_set_spte()")
Cc: stable@vger.kernel.org
Signed-off-by: Sean Christopherson <seanjc@google.com>