readahead: use folio_nr_pages() instead of shift operation

folio_nr_pages() is faster helper function to get the number of pages when
NR_PAGES_IN_LARGE_FOLIO is enabled.

Link: https://lkml.kernel.org/r/20250710060451.3535957-1-chizhiling@163.com


Signed-off-by: Chi Zhiling <chizhiling@kylinos.cn>
Acked-by: David Hildenbrand <david@redhat.com>
Reviewed-by: Ryan Roberts <ryan.roberts@arm.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>