mm: remove PageReadahead

This flag is now only used on folios, so we can remove all the page
accessors.

Link: https://lkml.kernel.org/r/20240821193445.2294269-4-willy@infradead.orgSigned-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

include/linux/page-flags.h

@@ -553,8 +553,8 @@ PAGEFLAG(MappedToDisk, mappedtodisk, PF_NO_TAIL)
 /* PG_readahead is only used for reads; PG_reclaim is only for writes */
 PAGEFLAG(Reclaim, reclaim, PF_NO_TAIL)
 	TESTCLEARFLAG(Reclaim, reclaim, PF_NO_TAIL)
-PAGEFLAG(Readahead, readahead, PF_NO_COMPOUND)
-	TESTCLEARFLAG(Readahead, readahead, PF_NO_COMPOUND)
+FOLIO_FLAG(readahead, FOLIO_HEAD_PAGE)
+	FOLIO_TEST_CLEAR_FLAG(readahead, FOLIO_HEAD_PAGE)
 
 #ifdef CONFIG_HIGHMEM
 /*