mm/hugetlb_vmemmap: don't synchronize_rcu() without HVO

hugetlb_vmemmap_optimize_folio() and hugetlb_vmemmap_restore_folio() are
wrappers meant to be called regardless of whether HVO is enabled. 
Therefore, they should not call synchronize_rcu().  Otherwise, it
regresses use cases not enabling HVO.

So move synchronize_rcu() to __hugetlb_vmemmap_optimize_folio() and
__hugetlb_vmemmap_restore_folio(), and call it once for each batch of
folios when HVO is enabled.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: bd225530a4c7 ("mm/hugetlb_vmemmap: fix race with speculative PFN walkers")
Signed-off-by: Yu Zhao <[email protected]>
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-lkp/[email protected]
Reported-by: Janosch Frank <[email protected]>
Tested-by: Marc Hartmayer <[email protected]>
Acked-by: Muchun Song <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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