s390/time: steer clocksource on STP sync events

On STP sync events the TOD clock will jump in time, either forward or
backward. The TOD clocksource claims to be continuous but in case of
an STP sync with a negative offset it is not.

Subtract the offset injected by the STP sync check from the result of
the TOD clocksource to make it continuous again. Add code to drift the
offset towards zero with a fixed rate, steering 1 second in ~9 hours.

Suggested-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>