SELinux: call capabilities code directory

For cleanliness and efficiency remove all calls to secondary-> and instead
call capabilities code directly.  capabilities are the only module that
selinux stacks with and so the code should not indicate that other stacking
might be possible.

Signed-off-by: Eric Paris <eparis@redhat.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>