ipmi/watchdog: fix watchdog timeout set on reboot

systemd by default starts watchdog on reboot and sets the timer to
ShutdownWatchdogSec=10min.  Reboot handler in ipmi_watchdog than reduces
the timer to 120s which is not enough time to boot a Xen machine with
a lot of RAM.  As a result the machine is rebooted the second time
during the long run of (XEN) Scrubbing Free RAM.....

Fix this by setting the timer to 120s only if it was previously
set to a low value.

Signed-off-by: Valentin Vidic <Valentin.Vidic@CARNet.hr>
Signed-off-by: Corey Minyard <cminyard@mvista.com>