s390/irq: enable irq sum accounting for /proc/stat again

s390/irq: enable irq sum accounting for /proc/stat again

For more than two years, since 27a91eec75109f0b3211867ee7948e9271739557
"/proc/stat: scalability of irq num per cpu" the output of /proc/stat is
broken.
The first field in the "intr" line should contain the sum of all interrupts,
however since the above mentioned change it is always zero.

The reason for that is that a per cpu irq sum variable had been introduced
which got incremented when calling kstat_incr_irqs_this_cpu(). However
on s390 we directly incremented only the per cpu per irq counter by accessing
the array element via kstat_cpu(smp_processor_id()).irqs[...].
So fix this and use the kstat_incr_irqs_this_cpu() wrapper which increments
both: the per cpu per irq counter and the per cpu irq sum counter.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>