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>

arch/s390/include/asm/vdso.h

@@ -33,6 +33,8 @@ struct vdso_data {
 	__u32 ectg_available;		/* ECTG instruction present	0x58 */
 	__u32 tk_mult;			/* Mult. used for xtime_nsec	0x5c */
 	__u32 tk_shift;			/* Shift used for xtime_nsec	0x60 */
+	__u32 ts_dir;			/* TOD steering direction	0x64 */
+	__u64 ts_end;			/* TOD steering end		0x68 */
 };
 
 struct vdso_per_cpu_data {