bcachefs: Use BTREE_ITER_PREFETCH in journal+btree iter

Introducing the journal+btree iter introduced a regression where we
stopped using BTREE_ITER_PREFETCH - this is a performance regression on
rotating disks.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/recovery.c

@@ -187,7 +187,7 @@ void bch2_btree_and_journal_iter_init(struct btree_and_journal_iter *iter,
 {
 	memset(iter, 0, sizeof(*iter));
 
-	iter->btree = bch2_trans_get_iter(trans, id, pos, 0);
+	iter->btree = bch2_trans_get_iter(trans, id, pos, BTREE_ITER_PREFETCH);
 	bch2_journal_iter_init(&iter->journal, journal_keys, id, 0, pos);
 }