These changes from Eric W. Biederman introduced network namespaces in
2.6.14.

  commit 881d966b48b035ab3f3aeaae0f3d3f9b584f45b2
  commit e730c15519d09ea528b4d2f1103681fa5937c0e6

Later, YOSHIFUJI Hideaki made the nd_net member inaccessible.

  commit c346dca10840a874240c78efe3f39acf4312a1f2

The three states are,

  nons		- before namespaces
  nd_net	- namespaces with direct access to nd_net member
  dev_net	- access via dev_net	NEW

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In 2.6.31, nd_net is visible again, so we need to look for and use
dev_net when it's present instead of checking for nd_net first.