Avoid deriving ELEMENT_PER_LATCH from cacheline

• buffer pool has latches that protect access to pages.

• there is a latch per N pages.
    (check page_hash_table for more details)

• N is calculated based on the cacheline size.

• for example: if cacheline size is
    : 64 then 7 pages pointers + 1 latch can be hosted on the same cacheline
    : 128 then 15 pages pointers + 1 latch can be hosted on the same cacheline

• arm generally have wider cacheline so with arm 1 latch is used
    to access 15 pages vs with x86 1 latch is used to access 7 pages.
    Naturally, the contention is more with arm case.

• said patch help relax this contention by limiting the elements
  per cacheline to 7 (+ 1 latch slot).
    for wider-cacheline (say 128), the remaining 8 slots are kept empty.
    this ensures there are no 2 latches on the same cacheline to avoid
  latch level contention.

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Said patch has shown improvement in performance in range of 2-5%.