clocksource/wdtest: Print time values for short udelay(1)

When a pair of clocksource reads separated by a udelay(1) claim less than a
full microsecond of elapsed time, print the measured delay as part of the
splat.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/717a2ddf-a80f-490b-aa3a-4e4b74fa56ca@paulmck-laptop

Author: paulmckrcu

kernel/time/clocksource-wdtest.c

@@ -137,7 +137,8 @@ static int wdtest_func(void *arg)
 	udelay(1);
 	j2 = clocksource_wdtest_ktime.read(&clocksource_wdtest_ktime);
 	pr_info("--- tsc-like times: %lu - %lu = %lu.\n", j2, j1, j2 - j1);
-	WARN_ON_ONCE(time_before(j2, j1 + NSEC_PER_USEC));
+	WARN_ONCE(time_before(j2, j1 + NSEC_PER_USEC),
+		  "Expected at least 1000ns, got %lu.\n", j2 - j1);
 
 	/* Verify tsc-like stability with various numbers of errors injected. */
 	max_retries = clocksource_get_max_watchdog_retry();