mm: Add folio_try_get_rcu()

This is the equivalent of page_cache_get_speculative().  Also add
folio_ref_try_add_rcu (the equivalent of page_cache_add_speculative)
and folio_get_unless_zero() (the equivalent of get_page_unless_zero()).

The new kernel-doc attempts to explain from the user's point of view
when to use folio_try_get_rcu() and when to use folio_get_unless_zero(),
because there seems to be some confusion currently between the users of
page_cache_get_speculative() and get_page_unless_zero().

Reimplement page_cache_add_speculative() and page_cache_get_speculative()
as wrappers around the folio equivalents, but leave get_page_unless_zero()
alone for now.  This commit reduces text size by 3 bytes due to slightly
different register allocation & instruction selections.

Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
Acked-by: Vlastimil Babka <[email protected]>
Reviewed-by: William Kucharski <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Acked-by: Kirill A. Shutemov <[email protected]>
Acked-by: Mike Rapoport <[email protected]>

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