Add missing copyright notice to large_dnode tests

Missing copyright notices were noticed during the Illumos
RTI process. Add LLNS 2016 copyright based on original merge
date.

Reviewed-by: Giuseppe Di Natale <guss80@gmail.com>
Reviewed-by: Alek Pinchuk <apinchuk@datto.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Ned Bass <bass6@llnl.gov>
Closes #8435