mm: open-code PageTail in folio_flags() and const_folio_flags()

JIRA: https://issues.redhat.com/browse/RHEL-84184

This patch is a backport of the following upstream commit:
commit 4de22b2
Author: Matthew Wilcox (Oracle) <[email protected]>
Date:   Mon Nov 25 20:17:18 2024 +0000

    mm: open-code PageTail in folio_flags() and const_folio_flags()

    It is unsafe to call PageTail() in dump_page() as page_is_fake_head() will
    almost certainly return true when called on a head page that is copied to
    the stack.  That will cause the VM_BUG_ON_PGFLAGS() in const_folio_flags()
    to trigger when it shouldn't.  Fortunately, we don't need to call
    PageTail() here; it's fine to have a pointer to a virtual alias of the
    page's flag word rather than the real page's flag word.

    Link: https://lkml.kernel.org/r/[email protected]
    Fixes: fae7d83 ("mm: add __dump_folio()")
    Signed-off-by: Matthew Wilcox (Oracle) <[email protected]>
    Cc: Kees Cook <[email protected]>
    Cc: <[email protected]>
    Signed-off-by: Andrew Morton <[email protected]>

Signed-off-by: Rafael Aquini <[email protected]>

Author: Rafael Aquini

include/linux/page-flags.h

@@ -310,7 +310,7 @@ static const unsigned long *const_folio_flags(const struct folio *folio,
 {
 	const struct page *page = &folio->page;
 
-	VM_BUG_ON_PGFLAGS(PageTail(page), page);
+	VM_BUG_ON_PGFLAGS(page->compound_head & 1, page);
 	VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags), page);
 	return &page[n].flags;
 }
@@ -319,7 +319,7 @@ static unsigned long *folio_flags(struct folio *folio, unsigned n)
 {
 	struct page *page = &folio->page;
 
-	VM_BUG_ON_PGFLAGS(PageTail(page), page);
+	VM_BUG_ON_PGFLAGS(page->compound_head & 1, page);
 	VM_BUG_ON_PGFLAGS(n > 0 && !test_bit(PG_head, &page->flags), page);
 	return &page[n].flags;
 }