s390: fix machine check handling

Commit eb7e7d76

 "s390: Replace __get_cpu_var uses" broke machine check
handling.

We copy machine check information from per-cpu to a stack variable for
local processing. Next we should zap the per-cpu variable, not the
stack variable.

Signed-off-by: Sebastian Ott <sebott@linux.vnet.ibm.com>
Reviewed-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Acked-by: Christoph Lameter <cl@linux.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>