linux: Fix integer overflows in initrd size handling
    
    These could be triggered by a crafted filesystem with very large files.
    
    Fixes: CVE-2020-15707
    
    Signed-off-by: Colin Watson <cjwatson@debian.org>
    Reviewed-by: Jan Setje-Eilers <jan.setjeeilers@oracle.com>
    Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>