[ATM]: clip causes unregister hang

If Classical IP over ATM module is loaded, its neighbor table gets
populated when permanent neighbor entries are created; but these entries
are not flushed when the device is removed. Since the entry never gets
flushed the unregister of the network device never completes.

This version of the patch also adds locking around the reference to
the atm arp daemon to avoid races with events and daemon state changes.
(Note: barrier() was never really safe)

Bug-reference: http://bugzilla.kernel.org/show_bug.cgi?id=6295


Signed-off-by: Stephen Hemminger <shemminger@osdl.org>
Signed-off-by: David S. Miller <davem@davemloft.net>