Avoid updating the L2ARC device header unnecessarily

If we do not write any buffers to the cache device and the evict hand
has not advanced do not update the cache device header.

Reviewed-by: Brian Behlendorf <[email protected]>
Signed-off-by: George Amanakis <[email protected]>
Closes #11522 
Closes #11537

Author: gamanakis

module/zfs/arc.c

@@ -8925,6 +8925,7 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
 	l2arc_write_callback_t	*cb = NULL;
 	zio_t 			*pio, *wzio;
 	uint64_t 		guid = spa_load_guid(spa);
+	l2arc_dev_hdr_phys_t	*l2dhdr = dev->l2ad_dev_hdr;
 
 	ASSERT3P(dev->l2ad_vdev, !=, NULL);
 
@@ -9153,7 +9154,8 @@ l2arc_write_buffers(spa_t *spa, l2arc_dev_t *dev, uint64_t target_sz)
 		 * Although we did not write any buffers l2ad_evict may
 		 * have advanced.
 		 */
-		l2arc_dev_hdr_update(dev);
+		if (dev->l2ad_evict != l2dhdr->dh_evict)
+			l2arc_dev_hdr_update(dev);
 
 		return (0);
 	}