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: Jiang 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: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>