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>

MFC after: 3 days
Reviewed by: delphij
Differential Revision: https://reviews.freebsd.org/D28682