Merge branch 'netns-Optimise-netns-ID-lookups'

Guillaume Nault says:

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netns: Optimise netns ID lookups

Netns ID lookups can be easily protected by RCU, rather than by holding
a spinlock.

Patch 1 prepares the code, patch 2 does the RCU conversion, and finally
patch 3 stops disabling BHs on updates (patch 2 makes that unnecessary).
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Signed-off-by: David S. Miller <davem@davemloft.net>