[NET]: Add a network namespace tag to struct net_device

Please note that network devices do not increase the count
count on the network namespace.  The are inside the network
namespace and so the network namespace tag is in the nature
of a back pointer and so getting and putting the network namespace
is unnecessary.

Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: David S. Miller <davem@davemloft.net>