In commit 0b47c126 (MDEV-13542)
a few calls to mtr_t::memo_push() were moved before a write latch
on the page was acquired. This introduced a race condition:

1. is_block_dirtied() returned false to mtr_t::memo_push()
2. buf_page_t::write_complete() was executed, the block marked clean,
and a page latch released
3. The page latch was acquired by the caller of mtr_t::memo_push(),
and mtr_t::m_made_dirty was not set even though the block is in
a clean state.

The impact of this race condition is that crash recovery and backups
may fail.

btr_cur_latch_leaves(), btr_store_big_rec_extern_fields(),
btr_free_externally_stored_field(), trx_purge_free_segment():
Acquire the page latch before invoking mtr_t::memo_push().
This fixes the regression caused by MDEV-13542.

Side note: It would suffice to set mtr_t::m_made_dirty at the time
we set the MTR_MEMO_MODIFY flag for a block. Currently that flag is
unnecessarily set if a mini-transaction acquires a page latch on
a page that is in a clean state, and will not actually modify the block.
This may cause unnecessary acquisitions of log_sys.flush_order_mutex
on mtr_t::commit().

mtr_t::free(): If the block had been exclusively latched in this
mini-transaction, set the m_made_dirty flag so that the flush order mutex
will be acquired during mtr_t::commit(). This should have been part of
commit 4179f93d (MDEV-18976).
It was necessary to change mtr_t::free() so that
WriteOPT_PAGE_CHECKSUM::operator() would be able to avoid writing
checksums for freed pages.