net: hook socketpair() into LSM

Use the newly created LSM-hook for socketpair(). The default hook
return-value is 0, so behavior stays the same unless LSMs start using
this hook.

Acked-by: Serge Hallyn <serge@hallyn.com>
Signed-off-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Acked-by: David S. Miller <davem@davemloft.net>
Signed-off-by: James Morris <james.morris@microsoft.com>