rocksdb Fix ArenaTest.UnmappedAllocation for transparent huge pages

Summary: ... by allocating in increments (1MiB) smaller than the typical smallest huge page size of 2MiB.

Fixes #12351

Test Plan:

sudo bash -c 'echo "always" > /sys/kernel/mm/transparent_hugepage/enabled'

And see unit test fails before this change, passes after this change

Feb 23 '24 23:02 pdillinger

@pdillinger has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

Feb 23 '24 23:02 facebook-github-bot

Unfortunately, this doesn't fix an internal failure in this test that we are seeing.

Feb 26 '24 16:02 pdillinger

@pdillinger has updated the pull request. You must reimport the pull request before landing.

Feb 26 '24 21:02 facebook-github-bot

@pdillinger has imported this pull request. If you are a Meta employee, you can view this diff on Phabricator.

Feb 26 '24 21:02 facebook-github-bot

@pdillinger merged this pull request in facebook/rocksdb@41849210e9acaba9a0d5bf842cf321f9328a212b.

Feb 27 '24 00:02 facebook-github-bot