[S390] vmemmap: allocate struct pages before 1:1 mapping

We have seen an oops in an OOM situation, where show_mem tried to
access the struct page of a dcss segment. The vmemmap code has
already created the 1:1 mapping but failed allocating the struct
pages. In the OOM case, show_mem now walks the memory. It uses
pfn_valid to detect if it may access the struct page. In the case
described above, the mapping was established and pfn_valid returned
true. As the struct pages were not allocated, the kernel oopsed.

We have to ensure that we have created the struct pages, before we
add a mapping pointing to the pages.
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>

arch/s390/mm/vmem.c

@@ -236,10 +236,10 @@ static int vmem_add_mem(unsigned long start, unsigned long size)
 {
 	int ret;
 
-	ret = vmem_add_range(start, size);
+	ret = vmem_add_mem_map(start, size);
 	if (ret)
 		return ret;
-	return vmem_add_mem_map(start, size);
+	return vmem_add_range(start, size);
 }
 
 /*