netns: let net_generic take pointer-to-const args

This commit is same in nature as v2.6.37-rc1-755-g3654654; the network
namespace itself is not modified when calling net_generic, so the
parameter can be const.

Signed-off-by: Jan Engelhardt <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/net/netns/generic.h

···
 	void *ptr[0];
 };
 
-static inline void *net_generic(struct net *net, int id)
+static inline void *net_generic(const struct net *net, int id)
 {
 	struct net_generic *ng;
 	void *ptr;