mm: Move definition of MIN_MEMORY_BLOCK_SIZE to a header

The macro MIN_MEMORY_BLOCK_SIZE is currently defined twice in two .c
files, and I need it in a third one to fix a powerpc bug, so let's
first move it into a header

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Ingo Molnar <mingo@elte.hu>
diff --git a/arch/x86/mm/init_64.c b/arch/x86/mm/init_64.c
index d865c4a..bbaaa00 100644
--- a/arch/x86/mm/init_64.c
+++ b/arch/x86/mm/init_64.c
@@ -28,6 +28,7 @@
 #include <linux/poison.h>
 #include <linux/dma-mapping.h>
 #include <linux/module.h>
+#include <linux/memory.h>
 #include <linux/memory_hotplug.h>
 #include <linux/nmi.h>
 #include <linux/gfp.h>
@@ -895,8 +896,6 @@
 }
 
 #ifdef CONFIG_X86_UV
-#define MIN_MEMORY_BLOCK_SIZE   (1 << SECTION_SIZE_BITS)
-
 unsigned long memory_block_size_bytes(void)
 {
 	if (is_uv_system()) {