btrfs-progs: consolidate num_stripes setting for striping RAID levels
All striping RAID Levels use the same method to set the number of RAID
stripes, so consolidate it.
Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>