netxen: protect resource cleanup by rtnl lock

o context resources can be in used, while resource cleanup is in progress,
  during fw recover.
o Null pointer execption can occur in send_cmd_desc, if fw recovery
  module frees tx ring without rtnl lock.
o Same applies to ethtool register dump.
Signed-off-by: NAmit Kumar Salecha <amit.salecha@qlogic.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/netxen/netxen_nic_main.c

@@ -2240,7 +2240,9 @@ netxen_detach_work(struct work_struct *work)
 
 	netxen_nic_down(adapter, netdev);
 
+	rtnl_lock();
 	netxen_nic_detach(adapter);
+	rtnl_unlock();
 
 	status = NXRD32(adapter, NETXEN_PEG_HALT_STATUS1);