btrfs: raid56: use in_range where applicable

While at it use the opportunity to simplify find_logical_bio_stripe by
reducing the scope of 'stripe_start' variable and squash the
sector-to-bytes conversion on one line.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Nikolay Borisov <nborisov@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>