Fix two mdadm bugs
    
    1) Improve error message when trying to use --grow -n<X> on a Linear
       array
    2) Allow assembly of explicitly specified arrays, even if they are
       disabled in /etc/mdadm.conf
    
    Resolves: bz#1122146, bz#1124310
    
    Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>