btr_free_if_exists(): Always use the BUF_GET_POSSIBLY_FREED mode
when accessing pages, because due to MDEV-24589 the function
fil_space_t::set_stopping(true) can be called at any time during
the execution of this function.

mtr_t::m_freeing_tree: New data member for debugging purposes.

buf_page_get_low(): Assert that the BUF_GET mode is not being used
anywhere during the execution of btr_free_if_exists().

In all code related to freeing or allocating pages, we will add some
robustness, by making more use of BUF_GET_POSSIBLY_FREED and by
reporting an error instead of crashing in some cases of corruption.