Let us invoke the debug member functions of mtr_t directly.

mtr_t::memo_contains(): Change the parameter type to
const rw_lock_t&. This function cannot be invoked on
buf_block_t::lock.

The function mtr_t::memo_contains_flagged() is intended to be invoked
on buf_block_t* or rw_lock_t*, and it along with
mtr_t::memo_contains_page_flagged() are the way to check whether
a buffer pool page has been latched within a mini-transaction.