s390/mm: correct return value of pmd_pfn

Git commit 152125b7
"s390/mm: implement dirty bits for large segment table entries"
broke the pmd_pfn function, it changed the return value from
'unsigned long' to 'int'. This breaks all machine configurations
with memory above the 8TB line.

Cc: stable@vger.kernel.org # 3.17+
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>