[DO NOT MERGE] mm: BUG if filemap_alloc_folio gives us a folio with a non-NULL -...

[DO NOT MERGE] mm: BUG if filemap_alloc_folio gives us a folio with a non-NULL ->private

We've seen some instances where we call __filemap_get_folio and get back
one with a ->private value that is non-NULL. Let's have the allocator
bug if that happens.

For now, let's just put this into the testing kernel. We can let Willy
decide if he wants it in mainline.

URL: https://tracker.ceph.com/issues/55421
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Xiubo Li <xiubli@redhat.com>
Cc: Luís Henriques <lhenriques@suse.de>
Signed-off-by: Jeff Layton <jlayton@kernel.org>