mm, mem-hotplug: recalculate lowmem_reserve when memory hotplug occurs

mm, mem-hotplug: recalculate lowmem_reserve when memory hotplug occurs

Currently, memory hotplug calls setup_per_zone_wmarks() and
calculate_zone_inactive_ratio(), but doesn't call
setup_per_zone_lowmem_reserve().

It means the number of reserved pages aren't updated even if memory hot
plug occur.  This patch fixes it.

Signed-off-by: KOSAKI Motohiro <[email protected]>
Reviewed-by: KAMEZAWA Hiroyuki <[email protected]>
Acked-by: Mel Gorman <[email protected]>
Reviewed-by: Minchan Kim <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>