With large folios, it's now incidentally possible to end up with a
clean, uptodate folio in the page cache that doesn't have a bch_folio
attached, if a folio has to be split.

This patch fixes __bch2_truncate_folio() to check for this; other code
paths appear to handle it.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>