ext4 extent status tree LRU locking

[Dave Hansen]

I've got a test case which I intended to use to stress the VM a bit.  It
fills memory up with page cache a couple of times.  It essentially runs
30 or so cp's in parallel.

98% of my CPU is system time, and 96% of _that_ is being spent on the
spinlock in ext4_es_lru_add().  I think the LRU list head and its lock
end up being *REALLY* hot cachelines and are *the* bottleneck on this
test.  Note that this is _before_ we go in to reclaim and actually start
calling in to the shrinker.  There is zero memory pressure in this test.

I'm not sure the benefits of having a proper in-order LRU during reclaim
outweigh such a drastic downside for the common case.

Any thoughts?
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