execute kdump_post after do_default_action
    
    User complains that kdump_post script doesn't execute after mount
    failed. This happened since mount failure will trigger
    kdump-error-handler.service, and then start kdump-error-handler.sh.
    However in kdump-error-handler.sh it doesn't execute kdump_post.
    Hence add it in this patch.
    
    Surely the function do_kdump_post need be moved into kdump-lib-initramfs.sh
    to be a common function.
    
    v1->v2:
        Add a return value to do_kdump_post when invoked in kdump_error-handler.sh.
        And call do_kdump_post earlier than do_default_action, otherwise
        it may not execute if reboot/poweroff/halt.
    
    Signed-off-by: Baoquan He <bhe@redhat.com>
    Acked-by: Dave Young <dyoung@redhat.com>
    Acked-by: Meifei Huang <mhuang@redhat.com>