tools/testing/scatterlist: Fix test to compile and run

Add missing define of ALIGN_DOWN to make the test build and run.  In
addition, __sg_alloc_table_from_pages now support unaligned maximum
segment, so adapt the test result accordingly.

Fixes: 07da1223

 ("lib/scatterlist: Add support in dynamic allocation of SG table from pages")
Link: https://lore.kernel.org/r/20201115120623.139113-1-leon@kernel.org
Signed-off-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>