s390/mm: avoid using pmd_to_page for !USE_SPLIT_PMD_PTLOCKS

pmd_to_page() is only available if USE_SPLIT_PMD_PTLOCKS is defined.
The use of pmd_to_page in the gmap code can cause compile errors if
NR_CPUS is smaller than SPLIT_PTLOCK_CPUS. Do not use pmd_to_page
outside of USE_SPLIT_PMD_PTLOCKS sections.
Reported-by: Mike Frysinger <vapier@gentoo.org>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/pgtable.c

@@ -322,11 +322,12 @@ static int gmap_alloc_table(struct gmap *gmap, unsigned long *table,
 static unsigned long __gmap_segment_gaddr(unsigned long *entry)
 {
 	struct page *page;
-	unsigned long offset;
+	unsigned long offset, mask;
 
 	offset = (unsigned long) entry / sizeof(unsigned long);
 	offset = (offset & (PTRS_PER_PMD - 1)) * PMD_SIZE;
-	page = pmd_to_page((pmd_t *) entry);
+	mask = ~(PTRS_PER_PMD * sizeof(pmd_t) - 1);
+	page = virt_to_page((void *)((unsigned long) entry & mask));
 	return page->index + offset;
 }