Fix failure of devices refusing to add to raids using bitmaps
    
    This fixes an issue with devices failing to be added to a raid using
    bitmaps, due to trying to write the bitmap with mis-aligned buffers
    using O_DIRECT
    
    Resolves bz789898 (f16) bz791189 (f15)
    
    Signed-off-by: Jes Sorensen <Jes.Sorensen@redhat.com>