to save space time and complexity.
* grub-core/disk/raid6_recover.c (raid6_table1): Removed.
(raid6_table2): Likewise.
(powx): New array.
(powx_inv): Likewise.
(poly): New const.
(grub_raid_block_mul): Replace with ...
(grub_raid_block_mulx): ...this.
(grub_raid6_init_table): Rewritten.
(grub_raid6_recover): Use power of generator representation.