Optimize the block size used on ZFS cache devices as is already done

for data and log devices.

Reported by:	Dmitryy Makarov
Submitted by:	smh
Reviewed by:	gibbs
Approved by:	re (delphij)
MFC after:	2 weeks

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/arc.c

@@ -5387,6 +5387,8 @@ l2arc_add_vdev(spa_t *spa, vdev_t *vd)
 
 	ASSERT(!l2arc_vdev_present(vd));
 
+	vdev_ashift_optimize(vd);
+
 	/*
 	 * Create a new l2arc device entry.
 	 */