security: allow Kconfig to set default mmap_min_addr protection

Since it was decided that low memory protection from userspace couldn't
be turned on by default add a Kconfig option to allow users/distros to
set a default at compile time.  This value is still tunable after boot
in /proc/sys/vm/mmap_min_addr

Discussion:
http://www.mail-archive.com/linux-security-module@vger.kernel.org/msg02543.html

Signed-off-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>