rtnetlink: speedup rtnl_dump_ifinfo()

When handling large number of netdevice, rtnl_dump_ifinfo()
is very slow because it has O(N^2) complexity.

Instead of scanning one single list, we can use the 256 sub lists
of the dev_index hash table.

This considerably speedups "ip link" operations

Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

include/net/net_namespace.h

@@ -28,6 +28,10 @@ struct ctl_table_header;
 struct net_generic;
 struct sock;
 
+
+#define NETDEV_HASHBITS    8
+#define NETDEV_HASHENTRIES (1 << NETDEV_HASHBITS)
+
 struct net {
 	atomic_t		count;		/* To decided when the network
 						 *  namespace should be freed.