mm: read page_type using READ_ONCE

KCSAN complains about possible data races: while we check for a page_type
-- for example for sanity checks -- we might concurrently modify the
mapcount that overlays page_type.

Let's use READ_ONCE to avoid load tearing (shouldn't make a difference)
and to make KCSAN happy.

Likely, we might also want to use WRITE_ONCE for the writer side of
page_type, if KCSAN ever complains about that.  But we'll not mess with
that for now.

Note: nothing should really be broken besides wrong KCSAN complaints.  The
sanity check that triggers this was added in commit 68f0320824fa
("mm/rmap: convert folio_add_file_rmap_range() into
folio_add_file_rmap_[pte|ptes|pmd]()").  Even before that similar races
likely where possible, ever since we added page_type in commit
6e292b9be7f4 ("mm: split page_type out from _mapcount").

Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: David Hildenbrand <[email protected]>
Reported-by: kernel test robot <[email protected]>
Closes: https://lore.kernel.org/oe-lkp/[email protected]
Reviewed-by: Oscar Salvador <[email protected]>
Reviewed-by: Miaohe Lin <[email protected]>
Cc: Matthew Wilcox <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

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