KVM: VMX: Read Posted Interrupt "control" exactly once per loop iteration

Use READ_ONCE() when loading the posted interrupt descriptor control
field to ensure "old" and "new" have the same base value.  If the
compiler emits separate loads, and loads into "new" before "old", KVM
could theoretically drop the ON bit if it were set between the loads.

Fixes: 28b835d6

 ("KVM: Update Posted-Interrupts Descriptor when vCPU is preempted")
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-27-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>