Merge branch 'net-avoid-slow-rcu'

Eric Dumazet says:

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net: avoid slow rcu synchronizations in cleanup_net()

RTNL is a contended mutex, we prefer to expedite rcu synchronizations
in contexts we hold RTNL.

Similarly, cleanup_net() is a single threaded critical component and
should also use synchronize_rcu_expedited() even when not holding RTNL.

First patch removes a barrier with no clear purpose in ipv6_mc_down()
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Signed-off-by: David S. Miller <davem@davemloft.net>