netns: add dummy struct inside "struct net_generic"

This is precursor to fixing "[id - 1]" bloat inside net_generic().

Name "s" is chosen to complement name "u" often used for dummy unions.

Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>