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 <jengelh@medozas.de>
Signed-off-by: David S. Miller <davem@davemloft.net>