s390/mm: correct return value of pmd_pfn

Git commit 152125b7a882df36a55a8eadbea6d0edf1461ee7
"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>

arch/s390/include/asm/pgtable.h

@@ -494,7 +494,7 @@ static inline int pmd_large(pmd_t pmd)
 	return (pmd_val(pmd) & _SEGMENT_ENTRY_LARGE) != 0;
 }
 
-static inline int pmd_pfn(pmd_t pmd)
+static inline unsigned long pmd_pfn(pmd_t pmd)
 {
 	unsigned long origin_mask;