net/netlabel: Add list_next_rcu() in rcu_dereference().

Add list_next_rcu() for fetching next list in rcu_deference safely.

Found with sparse in linux-next tree on tag next-20171116.

Signed-off-by: Tim Hansen <devtimhansen@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>