hwclock: Prevent excessive drift values

hwclock: Prevent excessive drift values

Failure of CMOS battery can cause writing of excessive drift
values (up to many years per day).

This causes excessive hwclock adjustment next time, which may lead
to overflow in calculate_adjustment() (and hang before 4a44a54b).

Prevent this situation, check drift for limits and reset drift to zero
instead.

Steps to reproduce:

mv /etc/adjtime /etc/adjtime.backup

rm /etc/adjtime
hwclock --set --date 2001-01-01\ 01:00:00

changing of /etc/adjtime.
mv /etc/adjtime /etc/adjtime.saved
hwclock --set --date 2001-01-02\ 01:00:01
mv /etc/adjtime.saved /etc/adjtime

echo "======= The /etc/adjtime has a \"correct\" look:"
cat /etc/adjtime

hwclock --debug --systohc --utc
echo "======= The /etc/adjtime now has deeply failed drift value:"
cat /etc/adjtime

mv /etc/adjtime /etc/adjtime.saved
hwclock --set --date 2015-01-01\ 01:00:00
mv /etc/adjtime.saved /etc/adjtime

hwclock --debug --adjust
echo "======= And the last /etc/adjtime:"
cat /etc/adjtime

mv /etc/adjtime.backup /etc/adjtime
hwclock --systohc --utc

Signed-off-by: Stanislav Brabec <sbrabec@suse.cz>