selinux: Consolidate sockcreate_sid logic

Consolidate the basic sockcreate_sid logic into a single helper function
which allows us to do some cleanups in the related code.

Signed-off-by: Paul Moore <paul.moore@hp.com>
Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: James Morris <jmorris@namei.org>