btrfs-progs: consolidate setting of RAID1 stripes
    
    All 3 different RAID1 profiles use the same calculation mehod for the
    number of used stripes.
    
    Now that we do table lookups fo rmost of the allocation control
    parameters, we can consolidate all 3 RAID1 profiles into a single branch
    for the calculation.
    
    Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
    Signed-off-by: David Sterba <dsterba@suse.com>