s390/vtime: Use __this_cpu_read() / get rid of READ_ONCE()

s390/vtime: Use __this_cpu_read() / get rid of READ_ONCE()

do_account_vtime() runs always with interrupts disabled, therefore use
__this_cpu_read() instead of this_cpu_read() to get rid of a pointless
preempt_disable() / preempt_enable() pair.

Also there are no concurrent writers to the cpu time accounting fields
in lowcore. Therefore get rid of READ_ONCE() usages.

Reviewed-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>