selinux: Return an error code only as a constant in sidtab_insert()

* Return an error code without storing it in an intermediate variable.

* Delete the local variable "rc" and the jump label "out" which became
  unnecessary with this refactoring.

Signed-off-by: Markus Elfring <elfring@users.sourceforge.net>
Signed-off-by: Paul Moore <paul@paul-moore.com>