mm/huge_memory: use folio_mapcount() in zap_huge_pmd() sanity check

We want to limit the use of page_mapcount() to the places where it is
absolutely necessary.  Let's similarly check for folio_mapcount()
underflows instead of page_mapcount() underflows like we do in
zap_present_folio_ptes() now.

Instead of the VM_BUG_ON(), we should actually be doing something like
print_bad_pte().  For now, let's keep it simple and use WARN_ON_ONCE(),
performing that check independently of DEBUG_VM.

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Cc: Chris Zankel <[email protected]>
Cc: Hugh Dickins <[email protected]>
Cc: John Paul Adrian Glaubitz <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Max Filippov <[email protected]>
Cc: Miaohe Lin <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: Peter Xu <[email protected]>
Cc: Richard Chang <[email protected]>
Cc: Rich Felker <[email protected]>
Cc: Ryan Roberts <[email protected]>
Cc: Yang Shi <[email protected]>
Cc: Yin Fengwei <[email protected]>
Cc: Yoshinori Sato <[email protected]>
Cc: Zi Yan <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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