network namespaces for generic netlink

This is a repost of my earlier patches, now with
fixed semantics and locking. The first patch is
required to make RCU-iteration of the network
namespace list possible, which the second patch
requires.

The third patch is preparation for nl80211 netns
work, I'm happy to give that to John instead to
go via the wireless tree with the patch that will
make use of the now exported function, but it is
possible that other generic netlink users will
also require it once they being upgrading their
family to be netns aware.

johannes

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