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>

fs/bcachefs/fs-io.c

@@ -466,6 +466,7 @@ int bchfs_truncate(struct mnt_idmap *idmap,
 	ret = bch2_truncate_folio(inode, iattr->ia_size);
 	if (unlikely(ret < 0))
 		goto err;
+	ret = 0;
 
 	truncate_setsize(&inode->v, iattr->ia_size);