s390/nmi: add missing __pa/__va address conversion of extended save area

Add missing __pa/__va address conversion of machine check extended
save area designation, which is an absolute address.

Note: this currently doesn't fix a real bug, since virtual addresses
are indentical to physical ones.

Reported-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Tested-by: Vineeth Vijayan <vneethv@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>