Fix l2tp stat counter

The 32 bit counter was incorrectly interpreted as signed
when being converted to the 64 bit value used for the
sysfs output.

Signed-off-by: Dominik Heidler <dominik@heidler.eu>
Reviewed-by: Tim Niemeyer <tim@tn-x.org>
[rebased to master, rename patch and remove buildscript part]
Signed-off-by: Tim Niemeyer <tim@tn-x.org>