UBUNTU: SAUCE: LSM stacking: fixup stacking kconfig

UBUNTU: SAUCE: LSM stacking: fixup stacking kconfig

The stack configs in the base stacking patches are confusing and
separate the selinux/smack stacking from the other LSMs with an
"extreme" stacking entry which is extremely confusing.

Switch the "extreme" stacking to a select for mutually exclusive
LSMs, which provides a better explanation of what is happening.

Fixes: 6c5100029055 ("LSM: general but not extreme module stacking")
Signed-off-by: John Johansen <john.johansen@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>