In MDEV-12353, the calls to mtr_t::memo_modify_page()
were accidentally removed along with
mlog_open_and_write_index() and its callers.

Let us resurrect the function to enable better debug checks.

mtr_t::flag_modified(): Renamed from mtr_t::set_modified()
and made private.

mtr_t::set_modified(): Take const buf_block_t& as a parameter.

In several mtr_t member functions, replace const buf_page_t&
parameters with const buf_block_t&, so that we can pass the
parameter to set_modified().

mtr_t::modify(): Add a MTR_MEMO_MODIFY entry for a block that
is guaranteed to be modified in the mini-transaction.