TREE_SRCU slows hotplug by factor ~16

[Mike Galbraith]

Greetings,

Running Steven's hotplug stress script in tip w. CLASSIC_SRCU takes 55s
in my i4790 box, whereas TREE_SRCU takes over 16m.  (Master with the
same config does it in 39s.. but then lockdep isn't enabled in master)

        -Mike