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/20241125201721.2963278-1-willy@infradead.org


Fixes: fae7d834 ("mm: add __dump_folio()")
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Kees Cook <kees@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>