btrfs-progs: use minimal number of stripes from btrfs_raid_attr
    
    Both btrfs_raid_attr and btrfs_raid_profile define the minimal number of
    stripes for each raid profile.
    
    The difference is in btrfs_raid_attr the number of stripes is called
    devs_min and in btrfs_raid_profile its called min_stripes.
    
    Signed-off-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
    Signed-off-by: David Sterba <dsterba@suse.com>