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Convert the incoming page to a folio and use it throughout, saving
several calls to compound_head().  Also use 'pos' for file position
rather than the ambiguous 'offset' and convert 'length' to type size_t
in case we get some truly ridiculous sized folios in the future.  This
function should now be large-folio safe, but I may have missed
something.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andreas Gruenbacher <agruenba@redhat.com>