bcachefs: add safety checks in bch2_btree_node_fill()

	struct btree *b;

	u32 seq;

	BUG_ON(level + 1 >= BTREE_MAX_DEPTH);

	if (unlikely(level >= BTREE_MAX_DEPTH)) {

		int ret = bch2_fs_topology_error(c, "attempting to get btree node at level %u, >= max depth %u",

						 level, BTREE_MAX_DEPTH);

		return ERR_PTR(ret);

	}

	if (unlikely(!bkey_is_btree_ptr(&k->k))) {

		struct printbuf buf = PRINTBUF;

		bch2_bkey_val_to_text(&buf, c, bkey_i_to_s_c(k));

		int ret = bch2_fs_topology_error(c, "attempting to get btree node with non-btree key %s", buf.buf);

		printbuf_exit(&buf);

		return ERR_PTR(ret);

	}

	if (unlikely(k->k.u64s > BKEY_BTREE_PTR_U64s_MAX)) {

		struct printbuf buf = PRINTBUF;

		bch2_bkey_val_to_text(&buf, c, bkey_i_to_s_c(k));

		int ret = bch2_fs_topology_error(c, "attempting to get btree node with too big key %s", buf.buf);

		printbuf_exit(&buf);

		return ERR_PTR(ret);

	}

	/*

	 * Parent node must be locked, else we could read in a btree node that's

	 * been freed: