btr_free_root_check(), btr_free_if_exists():
Replace page_id, page_size with fil_space_t* and the
root page number.

btr_free_but_not_root(): Take fil_space_t* as a parameter,
or NULL if the operation is not going to be redo-logged.

btr_free(): Pass space=NULL to btr_free_but_not_root().