fence_scsi: always do sg_turs before registration
    
    If a node has been fenced, or its key has been forcibly removed from
    device(s) by another node, the victim will see "unit attention" from the
    devices if/when it attempts to re-register. This error is normally
    reported just once, but will cause registration to fail. We can safely
    avoid this by calling sg_turs prior to each attempt to register with a
    device.
    
    Resolves: rhbz#640343
    
    Signed-off-by: Ryan O'Hara <rohara@redhat.com>
    Reviewed-by: Lon Hohberger <lhh@redhat.com>