fs/stat: Reduce memory requirements for stat_open

[Stefan Bader]

When reading from /proc/stat we allocate a large buffer to maximise
the chances of the results being from a single run and thus internally
consistent.  This currently is sized at 128 * num_possible_cpus() which,
in the face of kernels sized to handle large configurations (256 cpus
plus), results in the buffer being an order-4 allocation or more.
When system memory becomes fragmented these cannot be guarenteed, leading
to read failures due to allocation failures.

There seem to be two issues in play here.  Firstly the allocation is
going to be vastly over sized in the common case, as we only consume the
buffer based on the num_online_cpus().  Secondly, regardless of size we
should not be requiring allocations greater than PAGE_ALLOC_COSTLY_ORDER
as allocations above this order are significantly more likely to fail.

The following patch addesses both of these issues.  Does that make sense
generally? It seemed to stop top complaining wildly for the reporter
at least.

-Stefan

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