net/smc: use rcu_barrier() on module unload

Add rcu_barrier() to make sure no RCU readers or callbacks are
pending when the module is unloaded.

Signed-off-by: Ursula Braun <ubraun@linux.ibm.com>
Signed-off-by: Karsten Graul <kgraul@linux.ibm.com>
Signed-off-by: David S. Miller <davem@davemloft.net>