bootmem: Move contig_page_data definition to bootmem.c/nobootmem.c

Now that bootmem.c and nobootmem.c are separate, it's cleaner to
define contig_page_data in each file than in page_alloc.c with #ifdef.
Move it.

This patch doesn't introduce any behavior change.

-v2: According to Andrew, fixed the struct layout.
-tj: Updated commit description.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
diff --git a/mm/nobootmem.c b/mm/nobootmem.c
index f220b8d..6a018e4 100644
--- a/mm/nobootmem.c
+++ b/mm/nobootmem.c
@@ -23,6 +23,11 @@
 
 #include "internal.h"
 
+#ifndef CONFIG_NEED_MULTIPLE_NODES
+struct pglist_data __refdata contig_page_data;
+EXPORT_SYMBOL(contig_page_data);
+#endif
+
 unsigned long max_low_pfn;
 unsigned long min_low_pfn;
 unsigned long max_pfn;