x86: quick TSC calibration, improve

- make sure the final TSC timestamp is reliable too
Signed-off-by: NIngo Molnar <mingo@elte.hu>

arch/x86/kernel/tsc.c

@@ -316,6 +316,12 @@ static unsigned long quick_pit_calibrate(void)
 		}
 		t2 = get_cycles();
 
+		/*
+		 * Make sure we can rely on the second TSC timestamp:
+		 */
+		if (!pit_expect_msb(--expect))
+			goto failed;
+
 		/*
 		 * Ok, if we get here, then we've seen the
 		 * MSB of the PIT decrement QUICK_PIT_ITERATIONS