bcachefs: fsck: Fix oops in key_visible_in_snapshot()

The normal fsck code doesn't call key_visible_in_snapshot() with an
empty list of snapshot IDs seen (the current snapshot ID will always be
on the list), but str_hash_repair_key() ->
bch2_get_snapshot_overwrites() can, and that's totally fine as long as
we check for it.

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

fs/bcachefs/fsck.c

@@ -728,14 +728,8 @@ static int snapshots_seen_update(struct bch_fs *c, struct snapshots_seen *s,
 static bool key_visible_in_snapshot(struct bch_fs *c, struct snapshots_seen *seen,
 				    u32 id, u32 ancestor)
 {
-	ssize_t i;
-
 	EBUG_ON(id > ancestor);
 
-	/* @ancestor should be the snapshot most recently added to @seen */
-	EBUG_ON(ancestor != seen->pos.snapshot);
-	EBUG_ON(ancestor != darray_last(seen->ids));
-
 	if (id == ancestor)
 		return true;
 
@@ -751,11 +745,8 @@ static bool key_visible_in_snapshot(struct bch_fs *c, struct snapshots_seen *see
 	 * numerically, since snapshot ID lists are kept sorted, so if we find
 	 * an id that's an ancestor of @id we're done:
 	 */
-
-	for (i = seen->ids.nr - 2;
-	     i >= 0 && seen->ids.data[i] >= id;
-	     --i)
-		if (bch2_snapshot_is_ancestor(c, id, seen->ids.data[i]))
+	darray_for_each_reverse(seen->ids, i)
+		if (*i != ancestor && bch2_snapshot_is_ancestor(c, id, *i))
 			return false;
 
 	return true;