Fix assertion in Persistent L2ARC

At the end of l2arc_evict() fix an assertion in the case that l2ad_hand
+ distance == l2ad_end.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #16202
Closes #16207