zfs: Avoid updating the L2ARC device header unnecessarily

zfs: Avoid updating the L2ARC device header unnecessarily

From openzfs-master 0ae184a6b commit message:
  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.

Cherry-picked from openzfs 0ae184a6baaf71e155e9b19af81b75474622ff58
Patch Author:   George Amanakis <gamanakis@gmail.com>

Approved by:		re (gjb)
Reviewed by:            delphij
Differential Revision:  https://reviews.freebsd.org/D28682

(cherry picked from commit 5dce03847fdc7bc6eb959282c0ae2117b1991746)
(cherry picked from commit edb9049c3a782a51a77bd81a478c28cd1745323c)