int befs_bt_read_node (struct super_block * sb, befs_data_stream * ds, befs_btree_node * node, befs_off_t node_off);
Calls befs_read_datastream to read in the indicated btree node and makes sure its header fields are in cpu byteorder, byteswapping if necessary.
node->bh must be NULL when this function called first time. Don't forget brelse(node->bh) after last call.
On success, returns BEFS_OK and *node contains the btree node that starts at node_off, with the node->head fields in cpu byte order.
On failure, BEFS_ERR is returned.