s390/time: simplify stp time syncs

The way we call do_adjtimex() today is broken. It has 0 effect, as
ADJ_OFFSET_SINGLESHOT (0x0001) in the kernel maps to !ADJ_ADJTIME
(in contrast to user space where it maps to  ADJ_OFFSET_SINGLESHOT |
ADJ_ADJTIME - 0x8001). !ADJ_ADJTIME will silently ignore all adjustments
without STA_PLL being active. We could switch to ADJ_ADJTIME or turn
STA_PLL on, but still we would run into some problems:

- Even when switching to nanoseconds, we lose accuracy.
- Successive calls to do_adjtimex() will simply overwrite any leftovers
  from the previous call (if not fully handled)
- Anything that NTP does using the sysctl heavily interferes with our
  use.
- !ADJ_ADJTIME will silently round stuff > or < than 0.5 seconds

Reusing do_adjtimex() here just feels wrong. The whole STP synchronization
works right now *somehow* only, as do_adjtimex() does nothing and our
TOD clock jumps in time, although it shouldn't. This is especially bad
as the clock could jump backwards in time. We will have to find another
way to fix this up.

As leap seconds are also not properly handled yet, let's just get rid of
all this complex logic altogether and use the correct clock_delta for
fixing up the clock comparator and keeping the sched_clock monotonic.

This change should have 0 effect on the current STP mechanism. Once we
know how to best handle sync events and leap second updates, we'll start
with a fresh implementation.

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