bcachefs: Fix open coded set_btree_iter_dontneed()

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

fs/bcachefs/alloc_foreground.c

@@ -377,7 +377,7 @@ static struct open_bucket *try_alloc_bucket(struct btree_trans *trans, struct bc
 
 	ob = __try_alloc_bucket(c, ca, b, watermark, a, s, cl);
 	if (!ob)
-		iter.path->preserve = false;
+		set_btree_iter_dontneed(&iter);
 err:
 	if (iter.trans && iter.path)
 		set_btree_iter_dontneed(&iter);
@@ -447,7 +447,7 @@ bch2_bucket_alloc_early(struct btree_trans *trans,
 
 		ob = __try_alloc_bucket(trans->c, ca, k.k->p.offset, watermark, a, s, cl);
 next:
-		citer.path->preserve = false;
+		set_btree_iter_dontneed(&citer);
 		bch2_trans_iter_exit(trans, &citer);
 		if (ob)
 			break;
@@ -502,7 +502,7 @@ static struct open_bucket *bch2_bucket_alloc_freelist(struct btree_trans *trans,
 			ob = try_alloc_bucket(trans, ca, watermark,
 					      alloc_cursor, s, k, cl);
 			if (ob) {
-				iter.path->preserve = false;
+				set_btree_iter_dontneed(&iter);
 				break;
 			}
 		}