virtio/s390: fix race on airq_areas[]

The access to airq_areas was racy ever since the adapter interrupts got
introduced to virtio-ccw, but since commit 39c7dcb1

 ("virtio/s390:
make airq summary indicators DMA") this became an issue in practice as
well. Namely before that commit the airq_info that got overwritten was
still functional. After that commit however the two infos share a
summary_indicator, which aggravates the situation. Which means
auto-online mechanism occasionally hangs the boot with virtio_blk.

Signed-off-by: Halil Pasic <pasic@linux.ibm.com>
Reported-by: Marc Hartmayer <mhartmay@linux.ibm.com>
Reviewed-by: Cornelia Huck <cohuck@redhat.com>
Cc: stable@vger.kernel.org
Fixes: 96b14536

 ("virtio-ccw: virtio-ccw adapter interrupt support.")
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>