[NET] link_watch: Move link watch list into net_device

These days the link watch mechanism is an integral part of the
network subsystem as it manages the carrier status.  So it now
makes sense to allocate some memory for it in net_device rather
than allocating it on demand.

In fact, this is necessary because we can't tolerate a memory
allocation failure since that means we'd have to potentially
throw a link up event away.

It also simplifies the code greatly.

In doing so I discovered a subtle race condition in the use
of singleevent.  This race condition still exists (and is
somewhat magnified) without singleevent but it's now plugged
thanks to an smp_mb__before_clear_bit.

Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/linux/netdevice.h

@@ -467,6 +467,8 @@ struct net_device
 	/* device index hash chain */
 	struct hlist_node	index_hlist;
 
+	struct net_device	*link_watch_next;
+
 	/* register/unregister state machine */
 	enum { NETREG_UNINITIALIZED=0,
 	       NETREG_REGISTERED,	/* completed register_netdevice */