timekeeping: Update clocksource with stop_machine

timekeeping: Update clocksource with stop_machine

update_wall_time calls change_clocksource HZ times per second to check
if a new clock source is available. In close to 100% of all calls
there is no new clock. Replace the tick based check by an update done
with stop_machine.

Signed-off-by: Martin Schwidefsky <[email protected]>
Cc: Ingo Molnar <[email protected]>
Acked-by: John Stultz <[email protected]>
Cc: Daniel Walker <[email protected]>
LKML-Reference: <20090814134810.711836357@de.ibm.com>
Signed-off-by: Thomas Gleixner <[email protected]>