bcachefs: Fix truncate sometimes failing and returning 1

__bch_truncate_folio() may return 1 to indicate dirtyness of the folio
being truncated, needed for fpunch to get the i_size writes correct.

But truncate was forgetting to clear ret, and sometimes returning it as
an error.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>