readahead: convert readahead_expand() to use a folio

Replace the uses of page with a folio.  Also add a missing test for
workingset in the leading edge expansion.

Link: https://lkml.kernel.org/r/20230116193941.2148487-4-willy@infradead.org


Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Reviewed-by: William Kucharski <william.kucharski@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>