[S390] call nmi_enter/nmi_exit on machine checks

nmi_enter/nmi_exit includes the lockdep calls and various
other calls which were missing so far.

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