mm: only define hashdist variable when needed

For !CONFIG_NUMA, hashdist will always be 0, since it's setter is
otherwise compiled out.  So we can save 4 bytes of data and some .text
(although mostly in __init functions) by only defining it for
CONFIG_NUMA.
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Acked-by: David Rientjes <rientjes@google.com>
Reviewed-by: Michal Hocko <mhocko@suse.cz>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/linux/bootmem.h

@@ -357,12 +357,12 @@ extern void *alloc_large_system_hash(const char *tablename,
 /* Only NUMA needs hash distribution. 64bit NUMA architectures have
  * sufficient vmalloc space.
  */
-#if defined(CONFIG_NUMA) && defined(CONFIG_64BIT)
-#define HASHDIST_DEFAULT 1
+#ifdef CONFIG_NUMA
+#define HASHDIST_DEFAULT IS_ENABLED(CONFIG_64BIT)
+extern int hashdist;		/* Distribute hashes across NUMA nodes? */
 #else
-#define HASHDIST_DEFAULT 0
+#define hashdist (0)
 #endif
-extern int hashdist;		/* Distribute hashes across NUMA nodes? */
 
 
 #endif /* _LINUX_BOOTMEM_H */

mm/page_alloc.c

@@ -6013,9 +6013,9 @@ int percpu_pagelist_fraction_sysctl_handler(struct ctl_table *table, int write,
 	return ret;
 }
 
+#ifdef CONFIG_NUMA
 int hashdist = HASHDIST_DEFAULT;
 
-#ifdef CONFIG_NUMA
 static int __init set_hashdist(char *str)
 {
 	if (!str)