cnic: Fix panic in cnic_iscsi_nl_msg_recv() when device is down.

Some data structures are freed when the device is down and it will
crash if an ISCSI netlink message is received.  Add RCU protection
to prevent this.  In the shutdown path, ulp_ops[CNIC_ULP_L4] is
assigned NULL and rcu_synchronized before freeing the data
structures.

Signed-off-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>