lib/raid6: cleanup gen_syndrome function selection

Reorders functions in raid6_algos as well as the preference check
to reduce the number of functions tested on initialization.

Also, creates symmetry between choosing the gen_syndrome functions
and choosing the recovery functions.

Signed-off-by: Jim Kukunas <james.t.kukunas@linux.intel.com>
Signed-off-by: NeilBrown <neilb@suse.de>