mm: introduce helper function set_max_mapnr()

Introduce a helper function set_max_mapnr() to set global variable
max_mapnr.

Also unify condition compilation for max_mapnr with
CONFIG_NEED_MULTIPLE_NODES instead of CONFIG_DISCONTIGMEM.
Signed-off-by: NJiang Liu <jiang.liu@huawei.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Mauro Carvalho Chehab <mchehab@redhat.com>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>

include/linux/mm.h

@@ -25,8 +25,15 @@ struct file_ra_state;
 struct user_struct;
 struct writeback_control;
 
-#ifndef CONFIG_DISCONTIGMEM          /* Don't use mapnrs, do it properly */
+#ifndef CONFIG_NEED_MULTIPLE_NODES	/* Don't use mapnrs, do it properly */
 extern unsigned long max_mapnr;
+
+static inline void set_max_mapnr(unsigned long limit)
+{
+	max_mapnr = limit;
+}
+#else
+static inline void set_max_mapnr(unsigned long limit) { }
 #endif
 
 extern unsigned long totalram_pages;