btrfs-progs: simplify assignment of number of RAID stripes
    
    Simplify the assignment of the used number of RAID stripes in chunk
    allocation.
    
    For RAID levels 0, 5, 6 and 10 we first assigned it to the number of
    devices in the file-system and afterwards capped it to the upper bound of
    max_stripes. We can just use the max() macro for this.
    
    This will help in furhter refactorings.
    
    Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
    Signed-off-by: David Sterba <dsterba@suse.com>