timekeeping: Prevent false warning when persistent clock is not available

On arches with no persistent clock a message like this is printed during
boot:

[    0.000000] Persistent clock returned invalid value

The value is not invalid: Zero means that no persistent clock is available
and the absence of persistent clock should be quietly accepted.

Fixes: 3eca993740b8 ("timekeeping: Replace read_boot_clock64() with read_persistent_wall_and_boot_offset()")
Signed-off-by: Pavel Tatashin <pasha.tatashin@oracle.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: steven.sistare@oracle.com
Cc: daniel.m.jordan@oracle.com
Cc: sboyd@kernel.org
Cc: john.stultz@linaro.org
Link: https://lkml.kernel.org/r/20180725200018.23722-1-pasha.tatashin@oracle.com

kernel/time/timekeeping.c

@@ -1536,7 +1536,7 @@ void __init timekeeping_init(void)
 	if (timespec64_valid_strict(&wall_time) &&
 	    timespec64_to_ns(&wall_time) > 0) {
 		persistent_clock_exists = true;
-	} else {
+	} else if (timespec64_to_ns(&wall_time) != 0) {
 		pr_warn("Persistent clock returned invalid value");
 		wall_time = (struct timespec64){0};
 	}