[PATCH] percpu_counters: approximate but scalable counters

Several places in ext2 and ext3 are using filesystem-wide counters which use
global locking.  Mainly for the orlov allocator's heuristics.

To solve the contention which this causes we can trade off accuracy against
speed.

This patch introduces a "percpu_counter" library type in which the counts are
per-cpu and are periodically spilled into a global counter.  Readers only
read the global counter.

These objects are *large*.  On a 32 CPU P4, they are 4 kbytes.  On a 4 way
p3, 128 bytes.

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