btrfs: raid56: use fixed stripe length everywhere

The raid56 code assumes a fixed stripe length BTRFS_STRIPE_LEN but there
are functions passing it as arguments, this is not necessary. The fixed
value has been used for a long time and though the stripe length should
be configurable by super block member stripesize, this hasn't been
implemented and would require more changes so we don't need to keep this
code around until then.

Partially based on a patch from Qu Wenruo.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Tested-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Reviewed-by: Qu Wenruo <wqu@suse.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
[ update changelog ]
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>