KVM: Don't redo ktime_get() when calculating halt-polling stop/deadline

Calculate the halt-polling "stop" time using "start" instead of redoing
ktime_get().  In practice, the numbers involved are in the noise (e.g.,
in the happy case where hardware correctly predicts do_halt_poll and
there are no interrupts, "start" is probably only a few cycles old)
and either approach is perfectly ok.  But it's more precise to count
any extra latency toward the halt-polling time.

Reviewed-by: David Matlack <dmatlack@google.com>
Signed-off-by: Sean Christopherson <seanjc@google.com>
Message-Id: <20211009021236.4122790-17-seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>