Fix incorrect permissions on kdump dmesg file
    
    Also known as CVE-2021-20269. The kdump dmesg log files(kexec-dmesg.log,
    vmcore-dmesg.txt) are generated by shell redirection, which take the
    default umask value, making the files readable for group and others.
    
    This patch chmod these files, making them only accessible to owner.
    
    Signed-off-by: Tao Liu <ltao@redhat.com>
    Acked-by: Kairui Song <kasong@redhat.com>