s390/nmi: use irqentry_nmi_enter()/irqentry_nmi_exit()

With generic entry in place switch the nmi handler to use
the generic entry helper functions.

Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Alexander Gordeev <agordeev@linux.ibm.com>