jemalloc: touch backing pages for JemallocHugePageAllocator
jemalloc: touch backing pages for JemallocHugePageAllocator

Summary:
With THP set to madvise, page faults on these pages will block until a
huge page is found to service it. However, if memory becomes fragmented
before these pages are touched, then we end up blocking for kcompactd to
make a page available. As this increase pressure, oomd comes in and kills
us :(. So, preemptively touch these pages to get them backed as early as
possible to prevent stalling due to no available huge pages.

Reviewed By: hnaz, interwq

Differential Revision: D18579722

fbshipit-source-id: 5f54676a128207026cb16b9cdc03a055cbb1f9b9

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