s390/nmi: fix inline assembly constraints

Add missing memory clobbers / barriers or use the Q constraint where
possible to tell the compiler that the inline assemblies actually
access memory and not only pointers to memory.

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