There're two util headers under mm/ kselftest. Merge one with another. It
turns out util.h is the easy one to move.
When merging, drop PAGE_SIZE / PAGE_SHIFT because they're unnecessary
wrappers to page_size() / page_shift(), meanwhile rename them to psize()
and pshift() so as to not conflict with some existing definitions in some
test files that includes vm_util.h.
Signed-off-by: Peter Xu <peterx@xxxxxxxxxx>
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