s390/mm: ensure switch_mm() is executed with interrupts disabled

Architecture callback switch_mm() is allowed to be called with
enabled interrupts. However, our implementation of switch_mm()
does not expect that. Let's follow other architectures and make
sure switch_mm() is always executed with interrupts disabled,
regardless of what happens with the generic kernel code in the
future.

Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>