s390/mem_detect: fix lockdep irq tracing

When disabling and enabling interrupts we must tell lockdep.
So use local_irq_save()/restore() to disable and enable interrupts.
The DAT disabling/enabling get handled separately now.
Note: we may not call trace_hardirqs_on() with DAT disabled, since
the generic code may access vmalloc'ed data structures.

Reported-by: Michael Holzheu <holzheu@linux.vnet.ibm.com>
Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>