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>

include/linux/memory.h

@@ -20,6 +20,8 @@
 #include <linux/compiler.h>
 #include <linux/mutex.h>
 
+#define MIN_MEMORY_BLOCK_SIZE     (1 << SECTION_SIZE_BITS)
+
 struct memory_block {
 	unsigned long start_section_nr;
 	unsigned long end_section_nr;