s390/vtime: correct system time accounting

There is a slight misaccounting of system time in vtime_account_user.
This function is called once per HZ tick in interrupt context.
The irq_enter function already accounted the system time up to the
point of the irq_enter call. The system time from irq_enter until
vtime_account_user/do_account_vtime is reached is irq time but it
is accounted to the previous context.

Just drop the hardirq offset from arch/s390/kernel/vtime.c.

Reported-by: Frederic Weisbecker <fweisbec@gmail.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>