s390/irq/idle: Use stcke instead of stckf for time stamps

s390/irq/idle: Use stcke instead of stckf for time stamps

The upcoming cpu idle time accounting rework involves comparing and
subtracting cross cpu time stamps. Time stamps created with the stckf
instruction monotonic with respect to the local cpu. For cross cpu
monotonic time stamps the slightly slower stcke instruction has to
be used [1].

Convert the idle time accounting relevant usages of stckf to stcke.

[1] Principles of Operation - Setting and Inspecting the Clock

Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Acked-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>