Change license from GPLv2 to GPLv2+

This file only contains code from Red Hat, so it can use GPLv2+.
Tested with `git blame -M -C net/checksum.c`.

Signed-off-by: Stefan Weil <sw@weilnetz.de>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
