mm: util: move sysctls to mm/util.c

This moves all util related sysctls to mm/util.c, as part of the
kernel/sysctl.c cleaning, also removes redundant external
variable declarations and function declarations.

Signed-off-by: Kaixiong Yu <yukaixiong@huawei.com>
Reviewed-by: Kees Cook <kees@kernel.org>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Joel Granados <joel.granados@kernel.org>