kdumpctrl: kdump feasibility should fail if no crash memory
    
    Currently initramfs is rebuilt even when crash kernel memory is not
    available and then latter on kdump service is failed.
    
    Its better to fail during feasibility itself when crash memory is not
    reserved.
    
    Signed-off-by: Pratyush Anand <panand@redhat.com>
    Acked-by: Xunlei Pang <xlpang@redhat.com>
    Acked-by: Dave Young <dyoung@redhat.com>