tools/testing/scatterlist: Rejuvenate bit-rotten test

A couple small tweaks are needed to make the test build and run
on current kernels.

Link: https://lore.kernel.org/r/20201004154340.1080481-3-leon@kernel.org


Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>