raid6test: add new corner case for ioatdma driver

With 24 disks and an ioatdma instance with 16 source support there is a
corner case where the driver needs to be careful to account for the
number of implied sources in the continuation case.

Also bump the default case to test more than 16 sources now that it
triggers different paths in offload drivers.

Cc: Dave Jiang <dave.jiang@intel.com>
Acked-by: Dave Jiang <dave.jiang@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>