selinux: make mls_compute_sid always polyinstantiate

This patch removes the requirement that the new and related object types
differ in order to polyinstantiate by MLS level.  This allows MLS
polyinstantiation to occur in the absence of explicit type_member rules or
when the type has not changed.

Potential users of this support include pam_namespace.so (directory
polyinstantiation) and the SELinux X support (property polyinstantiation).

Signed-off-by: Eamon Walsh <ewalsh@tycho.nsa.gov>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: James Morris <jmorris@namei.org>