bcachefs: ensure iter->should_be_locked is set

Ensure that iter->should_be_locked is set to true before we
call bch2_trans_update in __bch2_dev_usrdata_drop.
Signed-off-by: Dan Robertson <dan@dlrobertson.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/migrate.c

@@ -73,7 +73,8 @@ static int __bch2_dev_usrdata_drop(struct bch_fs *c, unsigned dev_idx, int flags
 
 		bch2_btree_iter_set_pos(iter, bkey_start_pos(&sk.k->k));
 
-		ret   = bch2_trans_update(&trans, iter, sk.k, 0) ?:
+		ret   = bch2_btree_iter_traverse(iter) ?:
+			bch2_trans_update(&trans, iter, sk.k, 0) ?:
 			bch2_trans_commit(&trans, NULL, NULL,
 					BTREE_INSERT_NOFAIL);