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>

Patch-Name: linux-initrd-overflow.patch