[NET]: Add a network namespace parameter to struct sock

Sockets need to get a reference to their network namespace,
or possibly a simple hold if someone registers on the network
namespace notifier and will free the sockets when the namespace
is going to be destroyed.

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