int befs_btree_find (struct super_block * sb, befs_data_stream * ds, const char * key, befs_off_t * value);
On sucess, returns BEFS_OK and sets *value to the value stored with key (usually the disk block number of an inode).
On failure, returns BEFS_ERR or BEFS_BT_NOT_FOUND.
Read the superblock and rootnode of the b+tree. Drill down through the interior nodes using befs_find_key. Once at the correct leaf node, use befs_find_key again to get the actuall value stored with the key.