KVM: nVMX: Get to-be-acknowledge IRQ for nested VM-Exit at injection site

Move the logic to get the to-be-acknowledge IRQ for a nested VM-Exit from
nested_vmx_vmexit() to vmx_check_nested_events(), which is subtly the one
and only path where KVM invokes nested_vmx_vmexit() with
EXIT_REASON_EXTERNAL_INTERRUPT.  A future fix will perform a last-minute
check on L2's nested posted interrupt notification vector, just before
injecting a nested VM-Exit.  To handle that scenario correctly, KVM needs
to get the interrupt _before_ injecting VM-Exit, as simply querying the
highest priority interrupt, via kvm_cpu_has_interrupt(), would result in
TOCTOU bug, as a new, higher priority interrupt could arrive between
kvm_cpu_has_interrupt() and kvm_cpu_get_interrupt().

Unfortunately, simply moving the call to kvm_cpu_get_interrupt() doesn't
suffice, as a VMWRITE to GUEST_INTERRUPT_STATUS.SVI is hiding in
kvm_get_apic_interrupt(), and acknowledging the interrupt before nested
VM-Exit would cause the VMWRITE to hit vmcs02 instead of vmcs01.

Open code a rough equivalent to kvm_cpu_get_interrupt() so that the IRQ
is acknowledged after emulating VM-Exit, taking care to avoid the TOCTOU
issue described above.

Opportunistically convert the WARN_ON() to a WARN_ON_ONCE().  If KVM has
a bug that results in a false positive from kvm_cpu_has_interrupt(),
spamming dmesg won't help the situation.

Note, nested_vmx_reflect_vmexit() can never reflect external interrupts as
they are always "wanted" by L0.

Link: https://lore.kernel.org/r/20240906043413.1049633-3-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

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