KVM: VMX: Don't do full kick when triggering posted interrupt "fails"

Replace the full "kick" with just the "wake" in the fallback path when
triggering a virtual interrupt via a posted interrupt fails because the
guest is not IN_GUEST_MODE.  If the guest transitions into guest mode
between the check and the kick, then it's guaranteed to see the pending
interrupt as KVM syncs the PIR to IRR (and onto GUEST_RVI) after setting
IN_GUEST_MODE.  Kicking the guest in this case is nothing more than an
unnecessary VM-Exit (and host IRQ).

Opportunistically update comments to explain the various ordering rules
and barriers at play.

Signed-off-by: Sean Christopherson <[email protected]>
Message-Id: <[email protected]>
Signed-off-by: Paolo Bonzini <[email protected]>

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