fs/hugetlbfs/inode.c: mm/memory-failure.c: fix hugetlbfs hwpoison handling

has_extra_refcount() makes the assumption that the page cache adds a ref
count of 1 and subtracts this in the extra_pins case.  Commit a08c7193e4f1
(mm/filemap: remove hugetlb special casing in filemap.c) modifies
__filemap_add_folio() by calling folio_ref_add(folio, nr); for all cases
(including hugtetlb) where nr is the number of pages in the folio.  We
should adjust the number of references coming from the page cache by
subtracing the number of pages rather than 1.

In hugetlbfs_read_iter(), folio_test_has_hwpoisoned() is testing the wrong
flag as, in the hugetlb case, memory-failure code calls
folio_test_set_hwpoison() to indicate poison.  folio_test_hwpoison() is
the correct function to test for that flag.

After these fixes, the hugetlb hwpoison read selftest passes all cases.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: a08c7193e4f1 ("mm/filemap: remove hugetlb special casing in filemap.c")
Signed-off-by: Sidhartha Kumar <[email protected]>
Closes: https://lore.kernel.org/linux-mm/[email protected]/T/#m8e1469119e5b831bbd05d495f96b842e4a1c5519
Reported-by: Muhammad Usama Anjum <[email protected]>
Tested-by: Muhammad Usama Anjum <[email protected]>
Acked-by: Miaohe Lin <[email protected]>
Acked-by: Muchun Song <[email protected]>
Cc: James Houghton <[email protected]>
Cc: Jiaqi Yan <[email protected]>
Cc: Matthew Wilcox (Oracle) <[email protected]>
Cc: Naoya Horiguchi <[email protected]>
Cc: <[email protected]>	[6.7+]
Signed-off-by: Andrew Morton <[email protected]>

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