libata: don't check n_sectors during revalidation if zero

If the initial configuration fails early, n_sectors is left at zero.
Checking against it during revalidation makes retried configuration
fail due to n_sectors mismatch.  Ignore zero n_sectors during
revalidation.
Signed-off-by: NTejun Heo <htejun@gmail.com>
Signed-off-by: NJeff Garzik <jeff@garzik.org>

drivers/ata/libata-core.c

@@ -3700,7 +3700,8 @@ int ata_dev_revalidate(struct ata_device *dev, unsigned int readid_flags)
 		goto fail;
 
 	/* verify n_sectors hasn't changed */
-	if (dev->class == ATA_DEV_ATA && dev->n_sectors != n_sectors) {
+	if (dev->class == ATA_DEV_ATA && n_sectors &&
+	    dev->n_sectors != n_sectors) {
 		ata_dev_printk(dev, KERN_INFO, "n_sectors mismatch "
 			       "%llu != %llu\n",
 			       (unsigned long long)n_sectors,