mm/page_alloc: remove prefetchw() on freeing page to buddy system

The prefetchw() is introduced from an ancient patch[1].

The change log says:

    The basic idea is to free higher order pages instead of going
    through every single one.  Also, some unnecessary atomic operations
    are done away with and replaced with non-atomic equivalents, and
    prefetching is done where it helps the most.  For a more in-depth
    discusion of this patch, please see the linux-ia64 archives (topic
    is "free bootmem feedback patch").

So there are several changes improve the bootmem freeing, in which the
most basic idea is freeing higher order pages.  And as Matthew says,
"Itanium CPUs of this era had no prefetchers."

I did 10 round bootup tests before and after this change, the data doesn't
prove prefetchw() help speeding up bootmem freeing.  The sum of the 10
round bootmem freeing time after prefetchw() removal even 5.2% faster than
before.

[1]: https://lore.kernel.org/linux-ia64/[email protected]/

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Wei Yang <[email protected]>
Suggested-by: Matthew Wilcox <[email protected]>
Reviewed-by: Matthew Wilcox <[email protected]>
Reviewed-by: David Hildenbrand <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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