tipc: use RTNL lock to protect tipc_net_stop routine

As the tipc network initialization(ie, tipc_net_start routine) is
under RTNL protection, its corresponding deinitialization part(ie,
tipc_net_stop routine) should be protected by RTNL too.

Signed-off-by: Ying Xue <ying.xue@windriver.com>
Reviewed-by: Jon Maloy <jon.maloy@ericsson.com>
Reviewed-by: Erik Hugne <erik.hugne@ericsson.com>
Tested-by: Erik Hugne <erik.hugne@ericsson.com>
Signed-off-by: David S. Miller <davem@davemloft.net>