mm: remove PageSwapBacked

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

Link: https://lkml.kernel.org/r/20240821193445.2294269-3-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

@@ -528,9 +528,9 @@ PAGEFLAG(XenRemapped, xen_remapped, PF_NO_COMPOUND)
 PAGEFLAG(Reserved, reserved, PF_NO_COMPOUND)
 	__CLEARPAGEFLAG(Reserved, reserved, PF_NO_COMPOUND)
 	__SETPAGEFLAG(Reserved, reserved, PF_NO_COMPOUND)
-PAGEFLAG(SwapBacked, swapbacked, PF_NO_TAIL)
-	__CLEARPAGEFLAG(SwapBacked, swapbacked, PF_NO_TAIL)
-	__SETPAGEFLAG(SwapBacked, swapbacked, PF_NO_TAIL)
+FOLIO_FLAG(swapbacked, FOLIO_HEAD_PAGE)
+	__FOLIO_CLEAR_FLAG(swapbacked, FOLIO_HEAD_PAGE)
+	__FOLIO_SET_FLAG(swapbacked, FOLIO_HEAD_PAGE)
 
 /*
  * Private page markings that may be used by the filesystem that owns the page