KVM: nSVM: Clear EVENTINJ fields in vmcb12 on nested #VMEXIT

KVM: nSVM: Clear EVENTINJ fields in vmcb12 on nested #VMEXIT

According to the APM, from the reference of the VMRUN instruction:

  Upon #VMEXIT, the processor performs the following actions in order to
  return to the host execution context:

  ...

  clear EVENTINJ field in VMCB

KVM already syncs EVENTINJ fields from vmcb02 to cached vmcb12 on every
L2->L0  #VMEXIT. Since these fields are zeroed by the CPU on #VMEXIT, they
will mostly be zeroed in vmcb12 on nested #VMEXIT by nested_svm_vmexit().

However, this is not the case when:

  1. Consistency checks fail, as nested_svm_vmexit() is not called.
  2. Entering guest mode fails before L2 runs (e.g. due to failed load of
     CR3).

(2) was broken by commit 2d8a42be0e2b ("KVM: nSVM: synchronize VMCB
controls updated by the processor on every vmexit"), as prior to that
nested_svm_vmexit() always zeroed EVENTINJ fields.

Explicitly clear the fields in all nested #VMEXIT code paths.

Fixes: 3d6368ef580a ("KVM: SVM: Add VMRUN handler")
Fixes: 2d8a42be0e2b ("KVM: nSVM: synchronize VMCB controls updated by the processor on every vmexit")
Cc: stable@vger.kernel.org
Signed-off-by: Yosry Ahmed <yosry@kernel.org>
Link: https://patch.msgid.link/20260303003421.2185681-12-yosry@kernel.org
[sean: massage changelog formatting]
Signed-off-by: Sean Christopherson <seanjc@google.com>