SELinux: rename filename_compute_type argument to *type instead of *con

filename_compute_type() takes as arguments the numeric value of the type of
the subject and target.  It does not take a context.  Thus the names are
misleading.  Fix the argument names.

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