Fix incorrect file permissions of vmcore-dmesg-incomplete.txt
    
    vmcore-dmesg-incomplete.txt is generated by shell redirection,
    which taking the default umask value. When dmesg collector exits
    with non-zero, the file will exist and anyone can have access to
    it.
    
    This patch fixed the issue by chmod the file, making it accessible
    only to its owner.
    
    Signed-off-by: Tao Liu <ltao@redhat.com>
    Acked-by: Kairui Song <kasong@redhat.com>