selinux: provide socketpair callback

Make sure to implement the new socketpair callback so the SO_PEERSEC
call on socketpair(2)s will return correct information.

Acked-by: Serge Hallyn <serge@hallyn.com>
Acked-by: Stephen Smalley <sds@tycho.nsa.gov>
Signed-off-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: James Morris <james.morris@microsoft.com>